Previously, arrays were being incorrectly accepted as array indices.
This was because the isScalar() check only checked that the type was
not a vector or matrix, but still returned true for scalar arrays.

This patch changes the isScalar() check so that it returns false for
arrays. This makes usage of the term "scalar" more consistent in the
shader translator. Most of the code using isScalar() was compatible
with this change. Code in util.cpp that used to assume that isScalar()
doesn't care about arrayness is refactored to work with the new
behavior.

BUG=angleproject:2102
TEST=angle_unittests

Change-Id: I2a7f4c30fca7917d1099d0400efe3de859338b2a