[ARM] Extra MVE-related testing.

This adds some extra RUN lines to existing test files, to check that
things that worked in previous architecture versions haven't
accidentally stopped working in 8.1-M. Also we add some new tests: a
test of scalar floating point instructions that could be easily
confused with the similar-looking vector ones at assembly time, a test
of basic load/store/move access to the FP registers (which has to work
even in integer-only MVE); and one final check of the really obvious
case where turning off MVE should make sure MVE instructions really
are rejected.

Reviewers: dmgreen, samparker, SjoerdMeijer, t.p.northover

Subscribers: javed.absar, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62682

llvm-svn: 364293