[ARM][ParallelDSP] Replace SExt uses

As loads are combined and widened, we replaced their sext users
operands whereas we should have been replacing the uses of the sext.
I've added a load of tests, with only a few of them originally
causing assertion failures, the rest improve pattern coverage.

Differential Revision: https://reviews.llvm.org/D65740

llvm-svn: 368404