[ARM] A predicate cast of a predicate cast is a predicate cast

The adds some very basic folding of PREDICATE_CASTS, removing cases when they
are chained together. These would already be removed eventually, as these are
lowered to copies. This just allows it to happen earlier, which can help other
simplifications.

Differential Revision: https://reviews.llvm.org/D67591

llvm-svn: 372012