Even though vmv.v.x has a non constant scalar operand, we can still
rematerialize it because we have split register allocation between
vectors and scalars.
InlineSpiller will check to make sure that the scalar operand is live at
the point where the rematerialization occurs, so this won't extend any
scalar live ranges. However this also means we may not be able to
rematerialize in some cases, as shown in @vmv.v.x_needs_extended.
It might be worthwhile teaching InlineSpiller to extend scalar live
ranges in a future patch. I experimented with this locally and it
reduced spills on 531.deepsjeng_r by a further 3%.
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