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: 69d1ae4	Previous: cc0b571	Ratio
add-fold-worklist/create	2287000 ns (± 91909)	2245000 ns (± 59823)	1.02
add-fold-worklist/rewrite	3943000 ns (± 29871)	3943000 ns (± 62416)	1
add-fold-worklist-local/create	2234000 ns (± 50988)	2268000 ns (± 50000)	0.99
add-fold-worklist-local/rewrite	3272000 ns (± 46949)	3319000 ns (± 31736)	0.99
add-zero-worklist/create	2162000 ns (± 108046)	2235000 ns (± 108232)	0.97
add-zero-worklist/rewrite	2533000 ns (± 86356)	2508000 ns (± 31572)	1.01
add-zero-reuse-worklist/create	1842000 ns (± 89817)	1834000 ns (± 90486)	1.00
add-zero-reuse-worklist/rewrite	2080000 ns (± 45833)	2108000 ns (± 32722)	0.99
mul-two-worklist/create	2350000 ns (± 32311)	2248000 ns (± 109222)	1.05
mul-two-worklist/rewrite	5530000 ns (± 135262)	5544500 ns (± 143297)	1.00
add-fold-forwards/create	2272500 ns (± 110525)	2205000 ns (± 111151)	1.03
add-fold-forwards/rewrite	2981000 ns (± 40144)	2998000 ns (± 62583)	0.99
add-zero-forwards/create	2238000 ns (± 89519)	2291000 ns (± 81627)	0.98
add-zero-forwards/rewrite	1928000 ns (± 32322)	1903000 ns (± 92249)	1.01
add-zero-reuse-forwards/create	1900000 ns (± 86669)	1866500 ns (± 87239)	1.02
add-zero-reuse-forwards/rewrite	1579000 ns (± 34674)	1562000 ns (± 24410)	1.01
mul-two-forwards/create	2270000 ns (± 77911)	2189000 ns (± 55971)	1.04
mul-two-forwards/rewrite	3627000 ns (± 109176)	3612000 ns (± 88471)	1.00
add-zero-reuse-first/create	1917000 ns (± 104591)	1862000 ns (± 71721)	1.03
add-zero-reuse-first/rewrite	8000 ns (± 1837)	8000 ns (± 2593)	1
add-zero-lots-of-reuse-first/create	1932000 ns (± 86431)	1815000 ns (± 64825)	1.06
add-zero-lots-of-reuse-first/rewrite	809000 ns (± 25587)	785000 ns (± 22749)	1.03

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 😎