cmd/compile: prevent overflow in walkinrange

In the compiler frontend, walkinrange indiscriminately calls Int64()
on const CTINT nodes, even though Int64's return value is undefined
for anything over 2⁶³ (in practise, it'll return a negative number).

This causes the introduction of bad constants during rewrites of
unsigned expressions, which make the compiler reject valid Go
programs.

This change introduces a preliminary check that Int64() is safe to
call on the consts on hand. If it isn't, walkinrange exits without
doing any rewrite.

Fixes #27143

Change-Id: I2017073cae65468a521ff3262d4ea8ab0d7098d9
Reviewed-on: https://go-review.googlesource.com/130735
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -121,6 +121,17 @@ func (n *Node) Int64() int64 {
 	return n.Val().U.(*Mpint).Int64()
 }
 
+// CanInt64 reports whether it is safe to call Int64() on n.
+func (n *Node) CanInt64() bool {
+	if !Isconst(n, CTINT) {
+		return false
+	}
+
+	// if the value inside n cannot be represented as an int64, the
+	// return value of Int64 is undefined
+	return n.Val().U.(*Mpint).CmpInt64(n.Int64()) == 0
+}
+
 // Bool returns n as a bool.
 // n must be a boolean constant.
 func (n *Node) Bool() bool {