[X86] Mark vector STRICT_FP_ROUND as Legal instead of Custom.

The Custom handler doesn't do anything for these nodes anyway.

SelectionDAGISel won't mutate them if they are Legal or Custom.
X86 has custom code for mutating them due to missing isel patterns.
When the isel patterns are added Legal will be the right answer.
So go ahead a change it now since that's where we'll end up.