feat: add loop invariant code motion pass

Adds `licm` as the final post-lowering optimizer pass. Detects natural
loops via back-edges and dominator-tree analysis, then hoists
side-effect-free loop-invariant instructions into loop preheader blocks.
Runs after GVN and branch_fold so redundancies are resolved before
hoisting decisions are made.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bench