cmd/compile: compare size in dead store elimination

Only remove stores that is shadowed by another store with same or
larger size. Normally we don't need this check because we did check
the types, but unsafe pointer casting can get around it.

Fixes #16769.

Change-Id: I3f7c6c57807b590a2f735007dec6c65a4fa01a34
Reviewed-on: https://go-review.googlesource.com/27320


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>