This change adds support for ESSL3 variant of built-in function mix()
that takes last argument as a boolean vector. lerp() - HLSL equivalent
of mix() doesn't look to be supporting a variant that takes last
argument as a boolean vector so emulated it for HLSL.

BUG=angleproject:1006
TESTS=dEQP tests
Fixes "no matching overloaded function found " errors in below tests:
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common.mix_*_b*
(Note: These tests still fail because of constant expression issues)

Change-Id: I79b353933cb450516b8678b1fdaeabe60417e9a7
Reviewed-on: https://chromium-review.googlesource.com/271751Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>