[ARM] Cleanup ARM CGP isSupportedValue

isSupportedValue explicitly checked and accepted many types of value,
primarily for debugging reasons. Remove most of these checks and do a
bit of refactoring now that the pass is more stable. This also enables
ZExts to be sources, but this has very little practical benefit at the
moment extend instructions will still be introduced.

Differential Revision: https://reviews.llvm.org/D52080

llvm-svn: 342395