[PowerPC] Fix invalid lxvdsx optimization (PR25157)

PR25157 identifies a bug where a load plus a vector shuffle is
incorrectly converted into an LXVDSX instruction.  That optimization
is only valid if the load is of a doubleword, and in the noted case,
it was not.  This corrects that problem.

Joint patch with Eric Schweitz, who provided the bugpoint-reduced test
case.

llvm-svn: 250324