cmd/compile: allow ops to specify clobbering input registers

cmd/compile: allow ops to specify clobbering input registers

Same as clobbering fixed registers, but which register is clobbered
depends on which register was assigned to the input.

Add code similar to resultInArg0 processing that makes a register
copy before allowing the op to clobber the last available copy of a value.

(Will be used by subsequent CLs in this stack.)

Change-Id: I6bad88b2cb9ac3303d960ff0fb1611727292cfc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/680335
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Mark Freeman <mark@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>