[SVE]Clarify TypeSize comparisons in llvm/lib/Transforms

Use isKnownXY comparators when one of the operands can be with
scalable vectors or getFixedSize() for all the other cases.

This patch also does bug fixes for getPrimitiveSizeInBits by using
getFixedSize() near the places with the TypeSize comparison.

Differential Revision: https://reviews.llvm.org/D89703