[MemCpyOpt] Fix a variety of scalable-type crashes

This patch fixes a variety of crashes resulting from the `MemCpyOptPass`
casting `TypeSize` to a constant integer, whether implicitly or
explicitly.

Since the `MemsetRanges` requires a constant size to work, all but one
of the fixes in this patch simply involve skipping the various
optimizations for scalable types as cleanly as possible.

The optimization of `byval` parameters, however, has been updated to
work on scalable types in theory. In practice, this optimization is only
valid when the length of the `memcpy` is known to be larger than the
scalable type size, which is currently never the case. This could
perhaps be done in the future using the `vscale_range` attribute.

Some implicit casts have been left as they were, under the knowledge
they are only called on aggregate types. These should never be
scalably-sized.

Reviewed By: nikic, tra

Differential Revision: https://reviews.llvm.org/D109329