cmd/compile: improve PPC64 rules for AtomicLoad{8,32}
This adds a rule to avoid the zero extension after an AtomicLoad8
or AtomicLoad32 since the atomic load has already filled it with
zeros. This eliminates an instruction in a high use block in findObject
and the AtomicLoad8 appears many times within runtime.
Change-Id: I7e684bf73d3812110bd371e05b1aa44fa235fc9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/354029
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>