Opt compiler: Instruction simplification for HAdd, HNeg, HNot, HSub.

Under assumptions for the 'cost' of each IR (eg. neither HAdd nor HSub
are faster than the other), transformations are only applied if they
(locally) cannot degrade the quality of the graph. The code could be
extended to look at uses of the IRs and detect more opportunities for
optimisations.  The optimisations in this patch do not look at other
uses for their inputs.

Change-Id: Ib60dab007af30f43421ef5bb55db2ec32fb8fc0c