cmd/compile/internal/ssa: correct sign extension for consts on riscv64

Correct sign extension handling for consts on riscv64. This fixes a bug
in part exposed by CL 302609 - previously 64 bit consts were rewritten into
multiple 32 bit consts and the expansion would result in sign/zero extension
not being eliminated. With this change a MOVDconst with a 64 bit value can be
followed by a MOV{B,H,W}reg, which will be eliminated without actually
truncating to a smaller value.

Change-Id: I8d9cd380217466997b341e008a1f139bc11a0d51
Reviewed-on: https://go-review.googlesource.com/c/go/+/303350


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>