[SVE][MVT] Fixed-length vector MVT ranges
- Reordered MVT simple types to group scalable vector types together.
- New range functions in MachineValueType.h to only iterate over the fixed-length int/fp vector types.
- Stopped backends which don't support scalable vector types from iterating over scalable types.
Reviewers: sdesmalen, greened
Reviewed By: greened
Differential Revision: https://reviews.llvm.org/D66339