This change splits TIntermConstantUnion::foldUnary into two
functions:
    - foldUnaryWithDifferentReturnType:
        - Handles constant folding of operations where the return
          type has a different number of components compared to the
          operand type.
    - foldUnaryWithSameReturnType:
        - Handles constant folding of unary operations where the
          return type is the same as operand type.

BUG=angleproject:913
TEST=angle_unittests, dEQP Tests
(dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.*)

Change-Id: I675891138d4e17fd2390c03e9f710e0be0b7c7b6
Reviewed-on: https://chromium-review.googlesource.com/283464Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>