AArch64CollectLOH: Rewrite as block-local analysis.

Previously this pass was using up to 5% compile time in some cases which
is a bit much for what it is doing. The pass featured a full blown
data-flow analysis which in the default configuration was restricted to a
single block.

This rewrites the pass under the assumption that we only ever work on a
single block. This is done in a single pass maintaining a state machine
per general purpose register to catch LOH patterns.

Differential Revision: https://reviews.llvm.org/D27329

llvm-svn: 288561