cmd/compile: fix memcombine pass for big endian, > 1 byte elements
The shift amounts were wrong in this case, leading to miscompilation
of load combining.
Also the store combining was not triggering when it should.
Fixes #64468
Change-Id: Iaeb08972c5fc1d6f628800334789c6af7216e87b
Reviewed-on: https://go-review.googlesource.com/c/go/+/546355 Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com> Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>