[SplitKit] Cope with no live subranges in defFromParent

Following on from D87757 "[SplitKit] Only copy live lanes", it is
possible to split a live range at a point when none of its subranges
are live. This patch handles that case by inserting an implicit def
of the superreg.

Patch by Quentin Colombet!

Differential Revision: https://reviews.llvm.org/D88397