cmd/compile: combine negation and word sign extension on riscv64

Use NEGW to produce a negated and sign extended word, rather than doing
the same via two instructions:

   neg     t0, t0
   sext.w  a0, t0

Becomes:

   negw    t0, t0

Change-Id: I824ab25001bd3304bdbd435e7b244fcc036ef212
Reviewed-on: https://go-review.googlesource.com/c/go/+/652319
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

test/codegen/arithmetic.go

@@ -174,7 +174,7 @@ func AddAddSubSimplify(a, b, c int) int {
 }
 
 func NegToInt32(a int) int {
-	// riscv64: "NEG","MOVW"
+	// riscv64: "NEGW",-"MOVW"
 	r := int(int32(-a))
 	return r
 }