opt: omit unnecessary assignment casts for UDF return values

Previously, UDF return values were being assignment-casted to the return
type of the UDF function when the types were identical. These casts are
no longer created.

Epic: CRDB-20370

Fixes #93210

Release note: None

pkg/sql/opt/exec/execbuilder/testdata/udf

@@ -95,3 +95,42 @@ SELECT fetch_a_of_1_strict(NULL::INT)
 query T kvtrace
 SELECT fetch_a_of_2_strict(1, NULL::INT)
 ----
+
+# Regression test for #93210. Do not plan unnecessary assignment casts on the
+# return values of UDFs.
+statement ok
+CREATE TABLE t93210 (
+  a INT PRIMARY KEY
+);
+CREATE FUNCTION fn93210() RETURNS INT STABLE LANGUAGE SQL AS 'SELECT a FROM t93210';
+
+# The body of the UDF should have no assignment cast expressions.
+query T
+EXPLAIN (OPT, TYPES)
+SELECT fn93210()
+----
+values
+ ├── columns: fn93210:4(int)
+ ├── cardinality: [1 - 1]
+ ├── stable
+ ├── stats: [rows=1]
+ ├── cost: 0.02
+ ├── key: ()
+ ├── fd: ()-->(4)
+ ├── distribution: test
+ ├── prune: (4)
+ └── tuple [type=tuple{int}]
+      └── udf: fn93210 [type=int]
+           └── body
+                └── limit
+                     ├── columns: a:1(int!null)
+                     ├── cardinality: [0 - 1]
+                     ├── stats: [rows=1]
+                     ├── key: ()
+                     ├── fd: ()-->(1)
+                     ├── distribution: test
+                     ├── scan t93210
+                     │    ├── columns: a:1(int!null)
+                     │    ├── stats: [rows=1000]
+                     │    └── key: (1)
+                     └── const: 1 [type=int]

pkg/sql/opt/optbuilder/scalar.go

@@ -694,7 +694,7 @@ func (b *Builder) buildUDF(
 			// its type matches the function return type.
 			returnCol := physProps.Presentation[0].ID
 			returnColMeta := b.factory.Metadata().ColumnMeta(returnCol)
-			if returnColMeta.Type != f.ResolvedType() {
+			if !returnColMeta.Type.Identical(f.ResolvedType()) {
 				if !cast.ValidCast(returnColMeta.Type, f.ResolvedType(), cast.ContextAssignment) {
 					panic(sqlerrors.NewInvalidAssignmentCastError(
 						returnColMeta.Type, f.ResolvedType(), returnColMeta.Alias))