cmd/compile: don't move nilCheck operations during tighten

cmd/compile: don't move nilCheck operations during tighten

Nil checks need to stay in their original blocks. They cannot
be moved to a following conditionally-executed block.

Fixes #72860

Change-Id: Ic2d66cdf030357d91f8a716a004152ba4c016f77
Reviewed-on: https://go-review.googlesource.com/c/go/+/657715
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>