cmd/compile: make sure ascompatee walk lhs init statements

CL 281152 improved ascompatee by removing the call to safeExpr on lhs.
But we forgot that lhs int statements, if any, must be walked prior
saving subexpressions, which cause the bug in #45706.

Fixes #45706

Change-Id: I0064315056ef4ca92ebf3c332c2e3a9bb2b26f68
Reviewed-on: https://go-review.googlesource.com/c/go/+/312632


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>