cmd/asm: remove needless check for negative right shift

In the parser, the shift value is always a uint64.

Change-Id: I9b50295a9f7d174ed1f6f9baf78ec0ed43db417f
Reviewed-on: https://go-review.googlesource.com/11322Reviewed-by: Andrew Gerrand <adg@golang.org>

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

@@ -812,10 +812,8 @@ func (p *Parser) term() uint64 {
 		case lex.RSH:
 			p.next()
 			shift := p.term()
-			if shift < 0 {
-				p.errorf("negative right shift %d", shift)
-			}
-			if shift > 0 && value&(1<<63) != 0 {
+			// shift is a uint, so can never be negative.
+			if value&(1<<63) != 0 {
 				p.errorf("right shift with high bit set")
 			}
 			value >>= uint(shift)