[release-branch.go1.18] cmd/compile: revert "backport fix for #51840"

This reverts CL 405436 (commit e1b14f5).

Fixes #53883.
Updates #51840.

Change-Id: Ide5a9568a7ae5b449ef154c29b69699a7e4b3f6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/417616
Reviewed-by: David Chase <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>
Reviewed-by: Jenny Rakoczy <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: mdempsky

src/cmd/compile/internal/typecheck/stmt.go

@@ -127,23 +127,13 @@ func assign(stmt ir.Node, lhs, rhs []ir.Node) {
 
 	checkLHS := func(i int, typ *types.Type) {
 		lhs[i] = Resolve(lhs[i])
-		if base.Flag.G != 0 || base.Debug.Unified != 0 {
-			// New logic added in CL 403837 for Go 1.19, which only has -G=3 and unified IR.
-			if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Type() == nil {
-				base.Assertf(typ.Kind() == types.TNIL, "unexpected untyped nil")
+		if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Name().Ntype == nil {
+			if typ.Kind() != types.TNIL {
 				n.SetType(defaultType(typ))
-			}
-		} else {
-			// Original logic from Go 1.18, which is still needed for -G=0.
-			if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Name().Ntype == nil {
-				if typ.Kind() != types.TNIL {
-					n.SetType(defaultType(typ))
-				} else {
-					base.Errorf("use of untyped nil")
-				}
+			} else {
+				base.Errorf("use of untyped nil")
 			}
 		}
-
 		if lhs[i].Typecheck() == 0 {
 			lhs[i] = AssignExpr(lhs[i])
 		}