[RDA][ARM][LowOverheadLoops] Iteration count IT blocks

[RDA][ARM][LowOverheadLoops] Iteration count IT blocks

Change the way that we remove the redundant iteration count code in
the presence of IT blocks. collectLocalKilledOperands has been
introduced to scan an instructions operands, collecting the killed
instructions and then visiting them too. This is used to delete the
code in the preheader which calculates the iteration count. We also
track any IT blocks within the preheader and, if we remove all the
instructions from the IT block, we also remove the IT instruction.
isSafeToRemove is used to remove any redundant uses of the iteration
count within the loop body.

Differential Revision: https://reviews.llvm.org/D74975