[ARM][ParallelDSP] Enable multiple uses of loads

[ARM][ParallelDSP] Enable multiple uses of loads

When choosing whether a pair of loads can be combined into a single
wide load, we check that the load only has a sext user and that sext
also only has one user. But this can prevent the transformation in
the cases when parallel macs use the same loaded data multiple times.

To enable this, we need to fix up any other uses after creating the
wide load: generating a trunc and a shift + trunc pair to recreate
the narrow values. We also need to keep a record of which loads have
already been widened.

Differential Revision: https://reviews.llvm.org/D59215

llvm-svn: 356132