cmd/compile: use zero register instead of specialized *zero instructions

cmd/compile: use zero register instead of specialized *zero instructions

This lets us get rid of lots of specialized opcodes for storing zero.
Instead, use regular store opcodes that just happen to use the zero
register as one of their inputs.

Change-Id: I2902a6f9b0831cb598df45189ca6bb57221bef72
Reviewed-on: https://go-review.googlesource.com/c/go/+/633075
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>