Don't accept sampler operands for unary operators 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/376319 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>