[PowerPC] Remove LE patterns matching generic stores/loads to VSX permuting ops

This patch corresponds to:
https://reviews.llvm.org/D21409

The LXVD2X, LXVW4X, STXVD2X and STXVW4X instructions permute the two doublewords
in the vector register when in little-endian mode. Custom code ensures that the
necessary swaps are inserted for these. This patch simply removes the possibilty
that a load/store node will match one of these instructions in the SDAG as that
would not insert the necessary swaps.

llvm-svn: 282144