[X86] Make getZeroVector return floating point vectors in their native type on SSE2 and later.

isel used to require zero vectors to be canonicalized to a single
type to minimize the number of patterns needed to match. This is
 no longer required.

I plan to do this to integers too, but floating point was simpler
to start with. Integer has a complication where v32i16/v64i8 aren't
legal when the other 512-bit integer types are.

llvm-svn: 371325