The code used to incorrectly assert that the right-hand side of shift
should have the same signedness as the left-hand side. Instead simply
assert that both the lhs and rhs are integer typed, and also don't
rely on aliasing via union when accessing bit shift operands.

Also disallow constant folded bit shifts where the right hand side is
greater than 31. Shifting with values greater than the width of the
type has undefined results in both ESSL and C++.

BUG=chromium:648135
TEST=angle_unittests

Change-Id: I84a99abc55f0eeda549b4781e954d17ba7b87552
Reviewed-on: https://chromium-review.googlesource.com/389351Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>