[AMDGPU] Add MachineDCE pass after RenameIndependentSubregs

Detect dead lanes can create some dead defs. Then RenameIndependentSubregs
will break a REG_SEQUENCE which may use these dead defs. At this point
a dead instruction can be removed but we do not run a DCE anymore.

MachineDCE was only running before live variable analysis. The patch
adds a mean to preserve LiveIntervals and SlotIndexes in case it works
past this.

Differential Revision: https://reviews.llvm.org/D59626

llvm-svn: 357805