[analyzer][NFC] Inline ExprEngine::handleLVectorSplat()

It seems like ExprEngine::handleLVectorSplat() was used at only 2
places. It might be better to directly inline them for readability.

It seems like these cases were not covered by tests according to my
coverage measurement, so I'm adding tests as well, demonstrating that no
behavior changed.
Besides that, I'm handling CK_MatrixCast similarly to how the rest of
the unhandled casts are evaluated.

Differential Revision: https://reviews.llvm.org/D105125

Reviewed by: NoQ