[ARM] Identity shuffles are legal

[ARM] Identity shuffles are legal

Identity shuffles, of the form (0, 1, 2, 3, ...) are perfectly OK under MVE
(they essentially just become bitcasts). We were not catching that in the
existing set of what we considered legal though. On NEON, they would be covered
by vext's, but that is not generally available in MVE.

This uses ShuffleVectorInst::isIdentityMask which is a little odd to use here
but does what we want and prevents us from just rewriting what is the same
function.

Differential Revision: https://reviews.llvm.org/D68241

llvm-svn: 373446