Fix isShiftedInt and isShiftedUint for widths > 32.

Summary:
Previously we were doing 1 << S.  "1" is an int, so this doesn't work
when S >= 32.

This patch also adds some static_asserts to these functions to ensure
that we don't hit UB by shifting left too much.

Reviewers: rnk

Subscribers: llvm-commits, dylanmckay

Differential Revision: https://reviews.llvm.org/D22441

llvm-svn: 275719