feat(cse): use dominance to make CSE work across blocks

[regehr]

ok it looks like this bit is actually load-bearing:

  -- `domCtx` keeps a stable handle on the original context after
  -- `ctx` is shadowed by the mutable copy.
  let domCtx := ctx.raw

this is worrisome to me because I don't understand the consequences

[github-actions]

VeIR Benchmarks

Details

Benchmark suite	Current: 25ee983	Previous: 0fb06d1	Ratio
add-fold-worklist/create	2836000 ns (± 205304)	2363500 ns (± 142914)	1.20
add-fold-worklist/rewrite	3885000 ns (± 79735)	3862000 ns (± 89211)	1.01
add-fold-worklist-local/create	2761000 ns (± 208610)	2319000 ns (± 43764)	1.19
add-fold-worklist-local/rewrite	3270500 ns (± 88000)	3243000 ns (± 16956)	1.01
add-zero-worklist/create	2793000 ns (± 118063)	2370000 ns (± 67833)	1.18
add-zero-worklist/rewrite	2516000 ns (± 38760)	2440000 ns (± 110726)	1.03
add-zero-reuse-worklist/create	2415500 ns (± 228076)	1952000 ns (± 79423)	1.24
add-zero-reuse-worklist/rewrite	2155500 ns (± 114254)	2031000 ns (± 45637)	1.06
mul-two-worklist/create	2746000 ns (± 98043)	2318500 ns (± 113259)	1.18
mul-two-worklist/rewrite	5366000 ns (± 32086)	5460000 ns (± 67150)	0.98
add-fold-forwards/create	2741500 ns (± 231620)	2233000 ns (± 111607)	1.23
add-fold-forwards/rewrite	2908000 ns (± 82817)	2892000 ns (± 47901)	1.01
add-zero-forwards/create	2833000 ns (± 121403)	2138000 ns (± 94887)	1.33
add-zero-forwards/rewrite	1959000 ns (± 80556)	1853000 ns (± 20849)	1.06
add-zero-reuse-forwards/create	2358000 ns (± 172252)	1965000 ns (± 32277)	1.20
add-zero-reuse-forwards/rewrite	1624000 ns (± 124248)	1560000 ns (± 55053)	1.04
mul-two-forwards/create	2829000 ns (± 124011)	2218000 ns (± 110558)	1.28
mul-two-forwards/rewrite	3585000 ns (± 102734)	3577000 ns (± 74570)	1.00
add-zero-reuse-first/create	2368000 ns (± 168896)	1857500 ns (± 121827)	1.27
add-zero-reuse-first/rewrite	9000 ns (± 2180)	8000 ns (± 1925)	1.13
add-zero-lots-of-reuse-first/create	2407500 ns (± 216129)	1907500 ns (± 97143)	1.26
add-zero-lots-of-reuse-first/rewrite	900000 ns (± 87023)	764000 ns (± 49366)	1.18

This comment was automatically generated by workflow using github-action-benchmark.

[regehr]

NB: this PR cannot be merged until dominance is robust with respect to deleting instructions

[axelcool1234]

I'll make sure to unblock this soon John 😎