Add OpEndIT() for marking the end of OpIT blocks

In ARM we need to prevent code motion to the inside of an
IT block.  This was done using a GenBarrier() to mark the end, but
it wasn't obvious that this is what was happening.  This CL adds
an explicit OpEndIT() that takes the LIR of the OpIT for future
checks.

Bug: 13751744
Change-Id: If41d2adea1f43f11ebb3b72906bd308252ce3d01