[LiveVariables] Skip verification of kills inside bundles

LiveVariables does not examine the contents of bundles, so
MachineVerifier should not expect it to know about kill flags on
operands of instructions inside a bundle.

With this fix we can enable machine verification after running the
LiveVariables analysis. Doing this does not show any problems in
check-llvm in an LLVM_ENABLE_EXPENSIVE_CHECKS build.

Differential Revision: https://reviews.llvm.org/D110700