[PowerPC] Partially enable the ISEL expansion pass.

The pass to expand ISEL instructions into if-then-else sequences in patch D23630
is currently disabled. This patch partially enable it by always removing the
unnecessary ISELs (all registers used by the ISELs are the same one) and folding
the ISELs which have the same input registers into unconditional copies.

Differential Revision: https://reviews.llvm.org/D40497

llvm-svn: 320414