ARM64 maintains booleans in the low byte of registers. Upper parts
of that register are junk.
This rule is using all 32 bits of a boolean-containing register, which
is wrong. Change the rule to only look at the low bit.

Fixes #57211

Change-Id: Ibbef86b2be859df3d06d993db00e1231c481c428
Reviewed-on: https://go-review.googlesource.com/c/go/+/456556
Auto-Submit: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/456558
Reviewed-by: Than McIntosh <thanm@google.com>

Similar to CL 456556 but for ppc64 instead of arm64.

Change docs about how booleans are stored in registers for ppc64.
We now don't promise to keep the upper bits zeroed; they might be junk.

To test, I changed the boolean generation instructions (MOVBZload* and ISEL*
with boolean type) to OR in 0x100 to the result. all.bash still passed,
so I think nothing else is depending on the upper bits of booleans.

Update #57211

Change-Id: Ie66f8934a0dafa34d0a8c2a37324868d959a852c
Reviewed-on: https://go-review.googlesource.com/c/go/+/456437
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: KAMPANAT THUMWONG (KONG PC) <1992kongpc.kth@gmail.com>
Run-TryBot: Archana Ravindar <aravind5@in.ibm.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/456735
Reviewed-by: Than McIntosh <thanm@google.com>