Implement compile-time optimization for matrixCompMult().

$mat matrixCompMult($mat x, $mat y);
$hmat matrixCompMult($hmat x, $hmat y);

This required some minor changes to `evaluate_n_way_intrinsic_of_type`
to allow the inputs to be matrices instead of vectors. Fortunately, most
of the moving parts were already generic/flexible enough that this just
worked, but some explicit checks for `x.isVector` needed to become
`!x.isScalar()`, and `columns` needed to become `slotCount`.

Change-Id: I1e22ecad37a7e187a7171e1f590a720f07cf9832
Bug: skia:12034
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/414436
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
5 files changed