[ARM] MVE VPT Blocks

A minor iteration on the MVE VPT Block pass to enable more efficient VPT Block
code generation: consecutive VPT predicated statements, predicated on the same
condition, will be placed within the same VPT Block. This essentially is also
an exercise to write some more tests for the next step, which should be more
generic also merging instructions when they are not consecutive.

Differential Revision: https://reviews.llvm.org/D63711

llvm-svn: 364298