AArch64CollectLOH: Rewrite as block-local analysis.

Re-apply r288561: This time with a fix where the ADDs that are part of a
3 instruction LOH would not invalidate the "LastAdrp" state. This fixes
http://llvm.org/PR31361

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

This reverts commit 9e6cedb0a4f14364d6511597a9160305e7d34493.

llvm-svn: 291266