Rather than attempting to legalize 1 x float, make sure the SD ISel never

Rather than attempting to legalize 1 x float, make sure the SD ISel never
generates it.  Make MVT::Vector expand-only, and remove the code in
Legalize that attempts to legalize it.

The plan for supporting N x Type is to continually epxand it in ExpandOp
until it gets down to 2 x Type, where it will be scalarized into a pair of
scalars.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24482 91177308-0d34-0410-b5e6-96231b3b80d8