Optimizing: Speed up HEnvironment use removal

Removal of use records from HEnvironment vregs involved iterating over
potentially large linked lists which made compilation of huge methods
very slow. This patch turns use lists into doubly-linked lists, stores
pointers to the relevant nodes inside HEnvironment and subsequently
turns the removals into constant-time operations.

Change-Id: I0e1d4d782fd624e7b8075af75d4adf0a0634a1ee