cmd/compile: memcombine different size stores

cmd/compile: memcombine different size stores

This CL implements the TODO in combineStores to allow combining
stores of different sizes, as long as the total size aligns to
2, 4, 8.

Fixes #72832.

Change-Id: I6d1d471335da90d851ad8f3b5a0cf10bdcfa17c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/661855
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>