Optimizing: Speed up HInstruction use removal

Similarly to a previous commit on HEnvironment use removal, this patch
adds links from instructions to their respective inputs' use lists for
contant-time removal at the cost of doubling the size of input lists
(from one pointer per entry to two). Manual testing shows that this
significantly reduces the time required to transform HGraph to SSA
form for some huge methods.

Change-Id: I8dc3e4b0c48a50ac1481eb55c31093b99f4dc29f