[MachineCombiner] Fix initialisation of LastUpdate for incremental update.

Summary:
Fixes a bogus iterator resulting from the removal of a block's first instruction at the point that incremental update is enabled.

Patch by Paul Walker.

Reviewers: fhahn, Gerolf, efriedma, MatzeB

Reviewed By: fhahn

Subscribers: aemerson, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D38734

llvm-svn: 315502