[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

llvm-svn: 372099