[SVE][AArch64] Replace TypeSize comparisons with their integer equivalents

In many places in the AArch64 backend we are comparing TypeSize objects,
but in fact we are only ever expecting fixed width types. I've changed
all such comparisons to use their integer equivalents by replacing
calls to getSizeInBits() with getFixedSizeInBits(), etc.

Differential Revision: https://reviews.llvm.org/D89116