nir: Add a LCSAA-pass

nir: Add a LCSAA-pass

V2: Do a "depth first search" to convert to LCSSA

V3: Small comment fixup

V4: Rebase, adapt to removal of function overloads

V5: Rebase, adapt to relocation of nir to compiler/nir
    Still need to adapt to potential if-uses
    Work around nir_validate issue

V6 (Timothy):
 - tidy lcssa and stop leaking memory
 - dont rewrite the src for the lcssa phi node
 - validate lcssa phi srcs to avoid postvalidate assert
 - don't add new phi if one already exists
 - more lcssa phi validation fixes
 - Rather than marking ssa defs inside a loop just mark blocks inside
   a loop. This is simpler and fixes lcssa for intrinsics which do
   not have a destination.
 - don't create LCSSA phis for loops we won't unroll
 - require loop metadata for lcssa pass
 - handle case were the ssa defs use outside the loop is already a phi

V7: (Timothy)
- pass indirect mask to metadata call

v8: (Timothy)
- make convert to lcssa a helper function rather than a nir pass
- replace inside loop bitset with on the fly block index logic.
- remove lcssa phi validation special cases
- inline code from useless helpers, suggested by Jason.
- always do lcssa on loops, suggested by Jason.
- stop making lcssa phis special. Add as many source as the block
  has predecessors, suggested by Jason.

V9: (Timothy)
- fix regression with the is_lcssa_phi field not being initialised
  to false now that ralloc() doesn't zero out memory.

V10: (Timothy)
- remove extra braces in SSA example, pointed out by Topi

V11: (Timothy)
- add missing support for LCSSA phis in if conditions.

V12: (Timothy)
- small tidy up suggested by Jason.
- always create lcssa phi even if it just points to an lcssa
  phi from an inner loop

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