[X86] Add a DAG combine to replace vector loads feeding a v4i32->v2f64...

[X86] Add a DAG combine to replace vector loads feeding a v4i32->v2f64 CVTSI2FP/CVTUI2FP node with a vzload.

But only when the load isn't volatile.

This improves load folding during isel where we only have vzload
and scalar_to_vector+load patterns. We can't have full vector load
isel patterns for the same volatile load issue.

Also add some missing masked cvtsi2fp/cvtui2fp with vzload patterns.

llvm-svn: 364728