cmd/compile: correct comment for len check when make slice

CL 226737 optimizes len check when make slice. The comment that cap is
constrainted to [0, 2^31) is not quite true, it's 31 or 63 depends on
whether it's 32/64-bit systems.

Change-Id: I6f54e41827ffe4d0b67a44975da3ce07b2fabbad
Reviewed-on: https://go-review.googlesource.com/c/go/+/227803
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1352,7 +1352,8 @@ opswitch:
 				Fatalf("walkexpr: invalid index %v", r)
 			}
 
-			// cap is constrained to [0,2^31), so it's safe to do:
+			// cap is constrained to [0,2^31) or [0,2^63) depending on whether
+			// we're in 32-bit or 64-bit systems. So it's safe to do:
 			//
 			// if uint64(len) > cap {
 			//     if len < 0 { panicmakeslicelen() }