[Inline Spiller] Consider bundles when marking defs as dead

Fix bug where the code expects just a single MI, but a series of
bundled MIs need to be handled instead.

The semi-formed bundled are created by SplitKit for the case where
not all lanes are live (buildSingleSubRegCopy). Then the remat
kicks in, and since the values that are copied in the bundle do not
need to be preserved due to the remat (dead defs), all instructions
in the bundle should be marked as dead.

However, only the first one gets marked as dead, which causes the
verifier to complain later with error: "Live range continues after
dead def flag".

Differential Revision: https://reviews.llvm.org/D156999