cmd/compile: avoid extending when already sufficiently masked on riscv64

cmd/compile: avoid extending when already sufficiently masked on riscv64

Removes more than 2000 instructions from the Go binary on linux/risv64.

Change-Id: I6db3e3b1c93f29f00869adcba7c6192bfb90b25c
Reviewed-on: https://go-review.googlesource.com/c/go/+/426259
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>