cmd/compile: indexed loads/stores can't be faultOnNilArg0

cmd/compile: indexed loads/stores can't be faultOnNilArg0

Because of the index, these ops can't guarantee faulting if arg0 is nil.

Clean up the PPC64 index ops - they can't take a sym or an offset.

Noticed while debugging #37881. I don't think it is the cause, but I guess
there is a chance.
Update #37881

Change-Id: Ic22925250bf7b1ba64e3cea1a65638bc4bab390c
Reviewed-on: https://go-review.googlesource.com/c/go/+/224457
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>