cmd/compile: issue VarDef only for pointer-ful types

Use OpVarDef only when the variable being defined has pointers in it.
VarDef markers are only used for liveness analysis, and that only
runs on pointer-ful variables.

Fixes #53810

Change-Id: I09b0ef7ed31e72528916fe79325f80bbe69ff9b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/419320
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/export_test.go

@@ -72,6 +72,7 @@ func (TestFrontend) StringData(s string) *obj.LSym {
 }
 func (TestFrontend) Auto(pos src.XPos, t *types.Type) *ir.Name {
 	n := ir.NewNameAt(pos, &types.Sym{Name: "aFakeAuto"})
+	n.SetType(t)
 	n.Class = ir.PAUTO
 	return n
 }