cmd/compile: avoid zero extension after properly typed atomic operation on riscv64
LoweredAtomicLoad8 is implemented using MOVBU, hence it is already zero
extended. LoweredAtomicCas32 and LoweredAtomicCas64 return a properly
typed boolean.
Change-Id: Ie0acbaa19403d59c7e5f76d060cc13ee51eb7834
Reviewed-on: https://go-review.googlesource.com/c/go/+/428214 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>