[ARM] Permit auto-vectorization using MVE

With enough codegen complete, we can now correctly report the number and size
of vector registers for MVE, allowing auto vectorisation. This also allows FP
auto-vectorization for MVE without -Ofast/-ffast-math, due to support for IEEE
FP arithmetic and parity between scalar and vector FP behaviour.

Patch by David Sherwood.

Differential Revision: https://reviews.llvm.org/D63728

llvm-svn: 368529