[Hexagon] Eliminate subregisters from PHI nodes before pipelining

The pipeliner needs to remove instructions from the SlotIndexes
structure when they are deleted. Otherwise, the SlotIndexes map
has stale data, and an assert will occur when adding new
instructions.

This patch also changes the pipeliner to make the back-edge of
a loop carried dependence 1 cycle. The 1 cycle latency is added
to the anti-dependence that represents the back-edge. This
changes eliminates a couple of hacks added to the pipeliner to
handle the latency of the back-edge. It is needed to correctly
pipeline the test case for the sub-register elimination pass.

llvm-svn: 328113