Unary plus and minus used to be accepted with sampler operands.
Increment/decrement operators also now generate a clearer error
message if a sampler operand is supplied.

TEST=angle_unittests
BUG=angleproject:1480

Change-Id: I2c5165c4eaac7b023d96e46a177e36f6536b0125
Reviewed-on: https://chromium-review.googlesource.com/376319Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>