cmd/vet: allow shifts by amounts calculated using unsafe

cmd/vet: allow shifts by amounts calculated using unsafe

The real world code that inspired this fix,
from runtime/pprof/map.go:

	// Compute hash of (stk, tag).
	h := uintptr(0)
	for _, x := range stk {
		h = h<<8 | (h >> (8 * (unsafe.Sizeof(h) - 1)))
		h += uintptr(x) * 41
	}
	h = h<<8 | (h >> (8 * (unsafe.Sizeof(h) - 1)))
	h += uintptr(tag) * 41

Change-Id: I99a95b97cba73811faedb0b9a1b9b54e9a1784a3
Reviewed-on: https://go-review.googlesource.com/37574
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>