[ppc] Correctly compute the cost of loading 32/64 bit memory into VSR

VSX has instructions lxsiwax/lxsdx that can load 32/64 bit value into VSX register cheaply. That patch makes it known to memory cost model, so the vectorization of the test case in pr30990 is beneficial.

Differential Revision: https://reviews.llvm.org/D26713

llvm-svn: 288560