cmd/compile: use a single const MOV operand for riscv64

Most platforms only use a single MOV const operand - remove the MOV{B,H,W}const
operands from riscv64 and consistently use MOVDconst instead. The implementation
of all four is the same and there is no benefit gained from having multiple const
operands (in fact it requires a lot more rewrite rules).

Change-Id: I0ba7d7554e371a1de762ef5f3745e9c0c30d41ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/302610


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