compiler/parser: make 'used as value' error in 'if' init the same of gc

It also fixed a typo in a comment.

For #385

compiler/checker_expressions.go

@@ -2166,7 +2166,7 @@ func (tc *typechecker) checkCompositeLiteral(node *ast.CompositeLiteral, typ ref
 			var elemTi *typeInfo
 			if cl, ok := kv.Value.(*ast.CompositeLiteral); ok {
 				if ti.Type.Elem().Kind() == reflect.Ptr {
-					// The slice (or array) as element *T, so the value '{..}' must be
+					// The slice (or array) has element *T, so the value '{..}' must be
 					// replaced with '&T{..}'.
 					kv.Value = ast.NewUnaryOperator(cl.Pos(), ast.OperatorAddress, cl)
 					tc.checkCompositeLiteral(cl, ti.Type.Elem().Elem()) // []*T -> T (or [n]*T -> T)

compiler/parser.go

@@ -895,7 +895,7 @@ LABEL:
 				if a, ok := init.(*ast.Assignment); ok && a.Type == ast.AssignmentSimple {
 					panic(syntaxError(tok.pos, "assignment %s used as value", init))
 				}
-				panic(syntaxError(tok.pos, "%s used as value", init))
+				panic(syntaxError(tok.pos, "cannot use %s as value", init))
 			}
 			init = nil
 		}

test/compare/testdata/syntax/if_statement.go

@@ -44,12 +44,12 @@ func main() {
 	if f() {}
 	if <-ch {} // ERROR `non-bool <-ch (type int) used as if condition`
 	if <-chb {}
-	if ch <- 3 {} // ERROR `ch <- 3 used as value`
-	if i++ for {} // ERROR `syntax error: i++ used as value`
-	if i++ {} // ERROR `syntax error: i++ used as value`
-	if i-- {} // ERROR `syntax error: i-- used as value`
+	if ch <- 3 {} // ERROR `cannot use ch <- 3 as value`
+	if i++ for {} // ERROR `syntax error: cannot use i++ as value`
+	if i++ {} // ERROR `syntax error: cannot use i++ as value`
+	if i-- {} // ERROR `syntax error: cannot use i-- as value`
 	if i = 3 {} // ERROR `syntax error: assignment i = 3 used as value`
-	if i := 3 { _ = i } // ERROR `syntax error: i := 3 used as value`
+	if i := 3 { _ = i } // ERROR `syntax error: cannot use i := 3 as value`
 	if print(); {} // ERROR `syntax error: missing condition in if statement`
 	if <-ch; {} // ERROR `syntax error: missing condition in if statement`
 	if ch <- 3; {} // ERROR `syntax error: missing condition in if statement`