[ARM] Attempt to use whole register vmovs for MVE shuffles.

[ARM] Attempt to use whole register vmovs for MVE shuffles.

MVE doesn't have the range of shuffle instructions available in Neon. We
also cannot use the trick of cutting a difficult vector shuffle in half
to simplify things. Instead we need to be more careful about how we
lower shuffles.

This patch adds an extra combine that attempts to find "whole lane"
vmovs when lowering shuffles of smaller types. This helps us make some
shuffles a lot simpler, generating single lane movs for the parts that
can make use of it, falling back to the original shuffle for the rest.

Differential Revision: https://reviews.llvm.org/D69509