cmd/compile: riscv: use full type for SUB used for comparison

Otherwise the SUB is being inferred as a boolean, which means that if
the values being compared have the same least significant byte they may
be erroneously seen as equal.

Change-Id: I471f0048a787a07833206ed5e8e863a1dfcbd28a
Reviewed-on: https://review.gerrithub.io/352838


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>