cmd/vet: skip unreachable "if" and "case" code in shift check.

cmd/vet: skip unreachable "if" and "case" code in shift check.

Such dead code is legitimate when dealing with arch-specific
types (int, uint, uintptr).

The CL removes the majority of 'too small for shift' false positives
from such a code.

Change-Id: I62c5635a1d3774ab2d71d3d7056f0589f214cbe5
Reviewed-on: https://go-review.googlesource.com/38065
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>