[PowerPC] Generate lxsd instead of the ld->mtvsrd sequence for vector loads

This patch addresses:

- Implementation within PPCISelLowering.cpp to check if we should use direct
load into vector instructions (such as lxsd/lfd ) when the scalar_to_vector
function is used; which will allow us to catch as many cases of the
scalar_to_vector uses as possible to translate the ld->mtvsrd sequence into
lxsd.

- Test cases to exhibit the behaviour of emitting lxsd/lfd.

Patch by amyk

Differential revision: https://reviews.llvm.org/D49698

llvm-svn: 340037