[SVE][AArch64] Fix TypeSize warning in loop vectorization legality

The warning would fire when calling isDereferenceableAndAlignedInLoop
with a scalable load. Calling isDereferenceableAndAlignedInLoop with a
scalable load would result in the use of the now deprecated implicit
cast of TypeSize to uint64_t through the overloaded operator.

This patch fixes this issue by:

- no longer considering vector loads as candidates in
  canVectorizeWithIfConvert. This doesn't make sense in the context of
  identifying scalar loads to vectorize.

- making use of getFixedSize inside isDereferenceableAndAlignedInLoop --
  this removes the dependency on the deprecated interface, and will
  trigger an assertion error if the function is ever called with a
  scalable type.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D89798