[release-branch.go1.18] cmd/compile: fix boolean comparison on RISCV64

[release-branch.go1.18] cmd/compile: fix boolean comparison on RISCV64

Following CL 421457, for RISCV64.

May fix RISCV64 builds.

Updates #52788.
Updates #53397.

Change-Id: Ifc34658703d1e8b97665e7b862060152e3005d71
Reviewed-on: https://go-review.googlesource.com/c/go/+/405553
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/421460
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>