cmd/compile: remove types.Compare map bucket special cases

cmd/compile: remove types.Compare map bucket special cases

runtime.hmap never directly refers to the bucket type (it uses an
unsafe.Pointer), thus it shouldn't be possible to have infinite
recursion here.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-linux-amd64-longtest-race,gotip-linux-amd64-longtest-aliastypeparams
Change-Id: I457885e48bbc352acedae1c0c389078f39ed9d65
Reviewed-on: https://go-review.googlesource.com/c/go/+/619037
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>