cmd/compile: convert branch with zero to more optimal branch zero on riscv64

Convert BLT and BGE with a zero valued constant to BGTZ/BLTZ/BLEZ/BGEZ as
appropriate.

Removes over 4,500 instructions from the go binary on riscv64.

Change-Id: Icc266e968b126ba04863ec88529630a9dd44498b
Reviewed-on: https://go-review.googlesource.com/c/go/+/342849


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>