Fix incorrect TypeSize->uint64_t cast in InductionDescriptor::isInductionPHI

The code was relying upon the implicit conversion of TypeSize to
uint64_t and assuming the type in question was always fixed. However,
I discovered an issue when running the canon-freeze pass with some
IR loops that contains scalable vector types. I've changed the code
to bail out if the size is unknown at compile time, since we cannot
compute whether the step is a multiple of the type size or not.

I added a test here:

  Transforms/CanonicalizeFreezeInLoops/phis.ll

Differential Revision: https://reviews.llvm.org/D118696