[dev.simd] cmd/compile: accounts rematerialize ops's output reginfo

[dev.simd] cmd/compile: accounts rematerialize ops's output reginfo

This CL implements the check for rematerializeable value's output
regspec at its remateralization site. It has some potential problems,
please see the TODO in regalloc.go.

Fixes #70451.

Change-Id: Ib624b967031776851136554719e939e9bf116b7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/695315
Reviewed-by: David Chase <drchase@google.com>
TryBot-Bypass: David Chase <drchase@google.com>