[release-branch.go1.17] cmd/compile: correct type of pointer difference on RISCV64

Pointer comparison is lowered to the following on RISCV64

(EqPtr x y) => (SEQZ (SUB <x.Type> x y))

The difference of two pointers (the SUB) should not be pointer
type. Otherwise it can cause the GC to find a bad pointer.

Updates #51101.
Fixes #51199.

Change-Id: I7e73c2155c36ff403c032981a9aa9cccbfdf0f64
Reviewed-on: https://go-review.googlesource.com/c/go/+/385655


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 1ed30ca5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/386474