cmd/compile: inline memequal(x, const, sz) for small sizes

cmd/compile: inline memequal(x, const, sz) for small sizes

This CL adds late expanded memequal(x, const, sz) inlining for 2, 4, 8
bytes size. This PoC is using the same method as CL 248404.
This optimization fires about 100 times in Go compiler (1675 occurrences
reduced to 1574, so -6%).
Also, added unit-tests to codegen/comparisions.go file.

Updates #37275

Change-Id: Ia52808d573cb706d1da8166c5746ede26f46c5da
Reviewed-on: https://go-review.googlesource.com/c/go/+/328291
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Trust: David Chase <drchase@google.com>