cmd/compile: avoid unnecessary sign/zero extension for consts on riscv64

Sign extension for consts is unnecessary and zero extension for consts can be avoided
via casts. This removes over 16,000 instructions from the Go binary, in part because it
allows for better zero const absorbtion in blocks - for example,
`(BEQ (MOVBU (MOVBconst [0])) cond yes no)` now becomes `(BEQZ cond yes no)` when
this change is combined with existing rules.

Change-Id: I27e791bfa84869639db653af6119f6e10369ba3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/265041


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