cmd/compile: absorb NEG into branch when possible on riscv64

We can end up with this situation due to our equality tests being based on
'SEQZ (SUB x y)' - if x is a zero valued constant, 'SUB x y' can be converted
to 'NEG x'. When used with a branch the SEQZ can be absorbed, leading to
'BNEZ (NEG x)' where the NEG is redundant.

Removes around 1700 instructions from the go binary on riscv64.

Change-Id: I947a080d8bf7d2d6378ab114172e2342ce2c51db
Reviewed-on: https://go-review.googlesource.com/c/go/+/342850


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>