cmd/compile: allow huge rsh in constants arithmetic

Currently an expression like

var v = 0 >> 1000

is rejected by gc with a "stupid shift" error, while gotype
compiles it successfully.

As suggested by gri on the issue tracker, allow an rsh right
operand to be any valid uint value.

Fixes #11328

Change-Id: I6ccb3b7f842338d91fd26ae37dd4fa279d7fc440
Reviewed-on: https://go-review.googlesource.com/13777Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/mparith2.go

@@ -229,7 +229,7 @@ func mprshfixfix(a, b *Mpint) {
 	}
 
 	s := Mpgetfix(b)
-	if s < 0 || s >= Mpprec {
+	if s < 0 {
 		Yyerror("stupid shift: %d", s)
 		if a.Val.Sign() < 0 {
 			Mpmovecfix(a, -1)

test/const.go

@@ -19,6 +19,9 @@ const (
 	c3div2  = 3 / 2
 	c1e3    = 1e3
 
+	rsh1 = 1e100 >> 1000
+	rsh2 = 1e302 >> 1000
+
 	ctrue  = true
 	cfalse = !ctrue
 )
@@ -48,6 +51,8 @@ func ints() {
 	assert(c3div2 == 1, "3/2")
 	assert(c1e3 == 1000, "c1e3 int")
 	assert(c1e3 == 1e3, "c1e3 float")
+	assert(rsh1 == 0, "rsh1")
+	assert(rsh2 == 9, "rsh2")
 
 	// verify that all (in range) are assignable as ints
 	var i int