Implement unary operator folding in a similar way to binary operator
folding, so that the code is easier to understand.

TEST=dEQP-GLES3.functional.shaders.constant_expressions.*
BUG=angleproject:817

Change-Id: I069bdb38f965e1badb3e8f3f954386b205b7bb00
Reviewed-on: https://chromium-review.googlesource.com/275185Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>