Fix constant folding of subscripting non-square matrices. Previously
constant folding would offset the pointer into the matrix in multiples
of the number of columns, when it should offset the pointer in
multiples of the number of rows.

Also change the MalformedShaderTest so that it only succeeds if vector
swizzle is being checked correctly. Previously compilation would fail
in the test either way because the shader code contained a call to an
undefined function.

Also refactor indexing checks and constant folding so that constant
folding is done entirely separately from out-of-range checks. Bogus
comments are removed from the constant folding functions.

BUG=angleproject:1444
TEST=angle_unittests, angle_end2end_tests

Change-Id: I7073b38f759e9b3635ee05947df4f6d8e23a39d5
Reviewed-on: https://chromium-review.googlesource.com/360112Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>