cmd/compile: avoid sign extension after word arithmetic on riscv64

cmd/compile: avoid sign extension after word arithmetic on riscv64

These instructions already do sign extension on output, so we can get rid of it.

Note: (MOVWreg (MULW x y)) may araise from divisions by constant,
generic rules replace them with multiply and may produce (Rsh32x64 (Mul32 _ _) _).

Change-Id: I41bc9b519e38bc6027311de604dadb962cd0bbf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/429757
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>