cmd/asm: fix handling of negative shifts.

The change that "fixed" LSH was incorrect, and the fix for RSH was poor.
Make both use a correct, simple test: if the 64-bit value as a signed
integer is negative, it's an error.

Really fixes #11278.

Change-Id: I72cca03d7ad0d64fd649fa33a9ead2f31bd2977b
Reviewed-on: https://go-review.googlesource.com/11325Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/asm/internal/asm/parse.go

@@ -807,16 +807,17 @@ func (p *Parser) term() uint64 {
 		case lex.LSH:
 			p.next()
 			shift := p.factor()
-			// shift is a uint, so can never be negative.
+			if int64(shift) < 0 {
+				p.errorf("left shift with high bit set")
+			}
 			return value << shift
 		case lex.RSH:
 			p.next()
 			shift := p.term()
-			// shift is a uint, so can never be negative.
-			if value&(1<<63) != 0 {
+			if int64(shift) < 0 {
 				p.errorf("right shift with high bit set")
 			}
-			value >>= uint(shift)
+			value >>= shift
 		case '&':
 			p.next()
 			value &= p.factor()