Right-shifting a negative number should sign-extend according to the
ESSL 3.00.6 spec. Implement sign-extending right shift so that it
doesn't hit any undefined behavior in the C++ spec. Negative lhs
operands are now allowed for bit-shift right.

Also implement bit-shift left via conversion to unsigned integer, so
that it does not hit signed integer overflow. Negative lhs operands
are now allowed also for bit-shift left as well.

BUG=chromium:654103
TEST=angle_unittests

Change-Id: Iee241de9fd0d74c2f8a88219bddec690bb8e4db2
Reviewed-on: https://chromium-review.googlesource.com/395688
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>