[SVE][CodeGen] Add new EVT/MVT getFixedSizeInBits() functions

When we know that a particular type is always going to be fixed
width we have so far been writing code like this:

  getSizeInBits().getFixedSize()

Since we are doing this in quite a few places now it seems to make
sense to add a new helper function that allows us to replace
these calls with a single getFixedSizeInBits() call.

Differential Revision: https://reviews.llvm.org/D88649