[PowerPC] Folding XForm to DForm loads requires alignment for some DForm loads.

Going from XForm Load to DSForm Load requires that the immediate be 4 byte
aligned.
If we are not aligned we must leave the load as LDX (XForm).
This bug is causing a compile-time failure in the benchmark h264ref.

Differential Revision: https://reviews.llvm.org/D51988

llvm-svn: 343525