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: b11fbb8	Previous: a8086a5	Ratio
add-fold-worklist/create	1678000 ns (± 63097)	2191000 ns (± 116585)	0.77
add-fold-worklist/rewrite	2739000 ns (± 133798)	4025500 ns (± 99281)	0.68
add-fold-worklist-local/create	1712000 ns (± 42515)	2104000 ns (± 59879)	0.81
add-fold-worklist-local/rewrite	2401000 ns (± 31396)	3305000 ns (± 90508)	0.73
add-zero-worklist/create	1720000 ns (± 97465)	2234000 ns (± 109081)	0.77
add-zero-worklist/rewrite	1852500 ns (± 121917)	2750000 ns (± 105299)	0.67
add-zero-reuse-worklist/create	1455000 ns (± 69226)	1842000 ns (± 83972)	0.79
add-zero-reuse-worklist/rewrite	1605000 ns (± 80603)	2160000 ns (± 52555)	0.74
mul-two-worklist/create	1772500 ns (± 113401)	2231000 ns (± 55895)	0.79
mul-two-worklist/rewrite	4006500 ns (± 116351)	5644500 ns (± 283151)	0.71
add-fold-forwards/create	1622000 ns (± 76819)	2202000 ns (± 109662)	0.74
add-fold-forwards/rewrite	2173000 ns (± 19537)	3032000 ns (± 65213)	0.72
add-zero-forwards/create	1664000 ns (± 132990)	2265000 ns (± 96205)	0.73
add-zero-forwards/rewrite	1430500 ns (± 73123)	1987500 ns (± 93854)	0.72
add-zero-reuse-forwards/create	1463000 ns (± 65957)	1832000 ns (± 89057)	0.80
add-zero-reuse-forwards/rewrite	1195000 ns (± 13096)	1542000 ns (± 22638)	0.77
mul-two-forwards/create	1777000 ns (± 81470)	2198000 ns (± 41455)	0.81
mul-two-forwards/rewrite	2600000 ns (± 30491)	3767000 ns (± 172147)	0.69
add-zero-reuse-first/create	1436500 ns (± 58769)	1855500 ns (± 114257)	0.77
add-zero-reuse-first/rewrite	11000 ns (± 1736)	8500 ns (± 2543)	1.29
add-zero-lots-of-reuse-first/create	1371000 ns (± 64578)	1789000 ns (± 89329)	0.77
add-zero-lots-of-reuse-first/rewrite	625000 ns (± 22766)	803000 ns (± 57817)	0.78

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 😎