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 <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Trust: Lynn Boger <[email protected]>

Author: laboger

src/cmd/compile/internal/ssa/gen/PPC64.rules

@@ -1019,6 +1019,8 @@
 (MOVWZreg x:(MOVWZloadidx _ _ _)) => x
 (MOVWreg x:(MOVWload _ _)) => x
 (MOVWreg x:(MOVWloadidx _ _ _)) => x
+(MOVBZreg x:(Select0 (LoweredAtomicLoad8 _ _))) => x
+(MOVWZreg x:(Select0 (LoweredAtomicLoad32 _ _))) => x
 
 // don't extend if argument is already extended
 (MOVBreg x:(Arg <t>)) && is8BitInt(t) && isSigned(t) => x