Support constant folding of common built-ins This change adds constant folding support for unary common built-ins: abs, sign, floor, trunc, round, roundEven, ceil and fract. BUG=angleproject:913 TEST= dEQP tests dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common* (80 out of 210 tests started passing with this change) Change-Id: I06800ed0e03764c0f7aab6bcd45c4f122de5a3c1 Reviewed-on: https://chromium-review.googlesource.com/265394 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>