[PowerPC] Fix i64 to vector lowering on big endian

Lowering for scalar to vector would skip if current subtarget is big
endian and the scalar is larger or equal than 64 bits. However there's
some issue in implementation that SToVRHS may refer to SToVLHS's scalar
size if SToVLHS is present, which leads to some crash.o

Reviewed By: nemanjai, shchenz

Differential Revision: https://reviews.llvm.org/D105094