cmd/compile: fold negation into subtraction on riscv64

cmd/compile: fold negation into subtraction on riscv64

Fold negation into subtraction and avoid double negation.

This removes around 500 instructions from the Go binary on riscv64.

Change-Id: I4aac6c87baa2a0759b180ba87876d488a23df6d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/431105
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>