Applying field selection directly on matrices is not mentioned in ESSL
1.00 or 3.00 specs. Remove erroneous code that generated odd error
messages when a shader tried to apply certain kinds of field selection
on a matrix.

BUG=angleproject:1118
TEST=angle_unittests, dEQP-GLES3.functional.shaders.swizzles.*

Change-Id: I7bbf5d0cbaee3f21d20b830d904c0feef445dd78
Reviewed-on: https://chromium-review.googlesource.com/293190Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>