[DetectDeadLanes] Enable machine verification after this pass

Machine verification after DetectDeadLanes has been disabled since the
pass was first added in D18427, but I guess this was just due to copy-
and-paste. Enabling it does not show any problems in check-llvm in an
LLVM_ENABLE_EXPENSIVE_CHECKS build.

Differential Revision: https://reviews.llvm.org/D110689