nir: add a loop unrolling pass

nir: add a loop unrolling pass

V2:
- tidy ups suggested by Connor.
- tidy up cloning logic and handle copy propagation
 based of suggestion by Connor.
- use nir_ssa_def_rewrite_uses to fix up lcssa phis
  suggested by Connor.
- add support for complex loop unrolling (two terminators)
- handle case were the ssa defs use outside the loop is already a phi
- support unrolling loops with multiple terminators when trip count
  is know for each terminator

V3:
- set correct num_components when creating phi in complex unroll
- rewrite update remap table based on Jasons suggestions.
- remove unrequired extract_loop_body() helper as suggested by Jason.
- simplify the lcssa phi fix up code for simple loops as per Jasons suggestions.
- use mem context to keep track of hash table memory as suggested by Jason.
- move is_{complex,simple}_loop helpers to the unroll code
- require nir_metadata_block_index
- partially rewrote complex unroll to be simpler and easier to follow.

V4:
- use rzalloc() when creating nir_phi_src but not setting pred right away
 fixes regression cause by ralloc() no longer zeroing memory.

V5:
- simplify calling of complex_unroll()
- use new loop terminator fields to get the break/continue from blocks
  and simplify loop unrolling code
- handle slightly less trivial loop terminators. if branches can
  now have instructions but can only contain a single block.
- use nir print type IR snippets in unroll function descriptions
- add better explanation and variable for why we need to clone
  additional times when the second terminator it the limiting
  terminator.
- partially convert out of ssa before unrolling loops (suggested by Jason)

v6:
- remove unused nir_builder
- use Jasons new from ssa helper
- tidy/fixup cursor use
- unroll terminators that contain control flow correctly
- unroll complex loops with control flow before the terminators
  correctly

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>