colexecjoin: optimize building output on the left in merge joiner

This commit updates the way we're building output in the merge joiner
from the left input when building directly from the left batch (i.e. not
from the buffered group). There, we need to repeat a single tuple
`toAppend` times, so we do it in a loop. This commit adds the
optimization of using `Bytes.Copy` for the bytes-like types as well as
BCE for sliceable types.
```
MergeJoiner/rows=32-24                     29.3MB/s ± 3%  29.5MB/s ± 3%     ~     (p=0.684 n=10+10)
MergeJoiner/rows=512-24                    79.4MB/s ± 2%  77.8MB/s ± 3%   -1.91%  (p=0.043 n=10+10)
MergeJoiner/rows=4096-24                    192MB/s ± 2%   189MB/s ± 1%   -1.36%  (p=0.029 n=10+10)
MergeJoiner/rows=32768-24                   278MB/s ± 1%   275MB/s ± 0%   -1.30%  (p=0.000 n=10+10)
MergeJoiner/oneSideRepeat-rows=32-24       37.3MB/s ± 3%  38.0MB/s ± 2%   +1.78%  (p=0.029 n=10+10)
MergeJoiner/oneSideRepeat-rows=512-24       212MB/s ± 1%   215MB/s ± 2%   +1.42%  (p=0.003 n=9+10)
MergeJoiner/oneSideRepeat-rows=4096-24      765MB/s ± 4%   770MB/s ± 3%     ~     (p=0.436 n=10+10)
MergeJoiner/oneSideRepeat-rows=32768-24    1.22GB/s ± 2%  1.23GB/s ± 2%     ~     (p=0.393 n=10+10)
MergeJoiner/bothSidesRepeat-rows=32-24     22.7MB/s ± 2%  22.9MB/s ± 2%     ~     (p=0.203 n=9+10)
MergeJoiner/bothSidesRepeat-rows=512-24     102MB/s ± 4%   104MB/s ± 2%   +2.38%  (p=0.011 n=10+10)
MergeJoiner/bothSidesRepeat-rows=4096-24    117MB/s ± 1%   127MB/s ± 1%   +9.11%  (p=0.000 n=10+9)
MergeJoiner/bothSidesRepeat-rows=32768-24  59.2MB/s ± 1%  67.1MB/s ± 1%  +13.48%  (p=0.000 n=10+10)
```

Release note: None

pkg/col/coldata/bytes.go

@@ -211,9 +211,9 @@ func (b *Bytes) Window(start, end int) *Bytes {
 	}
 }
 
-// copy copies a single value from src at position srcIdx into position destIdx
+// Copy copies a single value from src at position srcIdx into position destIdx
 // of the receiver. It is faster than b.Set(destIdx, src.Get(srcIdx)).
-func (b *Bytes) copy(src *Bytes, destIdx, srcIdx int) {
+func (b *Bytes) Copy(src *Bytes, destIdx, srcIdx int) {
 	if buildutil.CrdbTestBuild {
 		if b.isWindow {
 			panic("copy is called on a window into Bytes")

pkg/col/coldata/bytes_test.go

@@ -122,7 +122,7 @@ func applyMethodsAndVerify(
 			destIdx := rng.Intn(n)
 			srcIdx := rng.Intn(sourceN)
 			debugString += fmt.Sprintf("(%d, %d)", destIdx, srcIdx)
-			b1.copy(b1Source, destIdx, srcIdx)
+			b1.Copy(b1Source, destIdx, srcIdx)
 			b2[destIdx] = append([]byte(nil), b2Source[srcIdx]...)
 		case copySlice, appendSlice:
 			// Generate a length-inclusive destIdx.

pkg/col/coldata/json.go

@@ -70,10 +70,10 @@ func (js *JSONs) Window(start, end int) *JSONs {
 	}
 }
 
-// copy copies a single value from src at position srcIdx into position destIdx
+// Copy copies a single value from src at position srcIdx into position destIdx
 // of the receiver.
-func (js *JSONs) copy(src *JSONs, destIdx, srcIdx int) {
-	js.Bytes.copy(&src.Bytes, destIdx, srcIdx)
+func (js *JSONs) Copy(src *JSONs, destIdx, srcIdx int) {
+	js.Bytes.Copy(&src.Bytes, destIdx, srcIdx)
 }
 
 // CopySlice copies srcStartIdx inclusive and srcEndIdx exclusive []byte values

pkg/col/coldata/vec_tmpl.go

@@ -210,7 +210,7 @@ func (m *memColumn) Copy(args SliceArgs) {
 							m.nulls.SetNull(i + args.DestIdx)
 						} else {
 							// {{if .IsBytesLike}}
-							toCol.copy(fromCol, i+args.DestIdx, selIdx)
+							toCol.Copy(fromCol, i+args.DestIdx, selIdx)
 							// {{else}}
 							v := fromCol.Get(selIdx)
 							// {{if .Sliceable}}
@@ -238,7 +238,7 @@ func (m *memColumn) Copy(args SliceArgs) {
 					//gcassert:bce
 					selIdx := sel[i]
 					// {{if .IsBytesLike}}
-					toCol.copy(fromCol, i+args.DestIdx, selIdx)
+					toCol.Copy(fromCol, i+args.DestIdx, selIdx)
 					// {{else}}
 					v := fromCol.Get(selIdx)
 					// {{if .Sliceable}}
@@ -285,7 +285,7 @@ func _COPY_WITH_REORDERED_SOURCE(_SRC_HAS_NULLS bool) { // */}}
 		// {{end}}
 		{
 			// {{if .IsBytesLike}}
-			toCol.copy(fromCol, destIdx, srcIdx)
+			toCol.Copy(fromCol, destIdx, srcIdx)
 			// {{else}}
 			v := fromCol.Get(srcIdx)
 			toCol.Set(destIdx, v)