feat(verifier): Add check that successors have the same parents

[math-fehr]

This adds two checks on regions:

• A graph region has at most one block (checked by asserting the first and last block are equal)
• The successors of a block are in the same region

[github-actions]

VeIR Benchmarks

Details

Benchmark suite	Current: fd10491	Previous: e0005b3	Ratio
add-fold-worklist/create	1892000 ns (± 46036)	1890000 ns (± 82875)	1.00
add-fold-worklist/rewrite	3680000 ns (± 41896)	3516000 ns (± 30434)	1.05
add-fold-worklist-local/create	1880000 ns (± 17743)	1921000 ns (± 75108)	0.98
add-fold-worklist-local/rewrite	3522000 ns (± 38399)	3345000 ns (± 30656)	1.05
add-zero-worklist/create	1864000 ns (± 78948)	1906000 ns (± 93003)	0.98
add-zero-worklist/rewrite	2331000 ns (± 28385)	2282500 ns (± 46006)	1.02
add-zero-reuse-worklist/create	1560000 ns (± 15758)	1564000 ns (± 28018)	1.00
add-zero-reuse-worklist/rewrite	1931000 ns (± 35520)	1879000 ns (± 25929)	1.03
mul-two-worklist/create	1876500 ns (± 93744)	1908000 ns (± 45497)	0.98
mul-two-worklist/rewrite	5158500 ns (± 17709)	5040000 ns (± 247290)	1.02
add-fold-forwards/create	1904000 ns (± 88189)	1897000 ns (± 96247)	1.00
add-fold-forwards/rewrite	2680000 ns (± 32098)	2651000 ns (± 33111)	1.01
add-zero-forwards/create	1886000 ns (± 69686)	1883000 ns (± 75744)	1.00
add-zero-forwards/rewrite	1734000 ns (± 25203)	1705000 ns (± 29513)	1.02
add-zero-reuse-forwards/create	1545000 ns (± 75761)	1595000 ns (± 79100)	0.97
add-zero-reuse-forwards/rewrite	1391000 ns (± 64442)	1366000 ns (± 58799)	1.02
mul-two-forwards/create	1893000 ns (± 86467)	1898000 ns (± 90461)	1.00
mul-two-forwards/rewrite	3239000 ns (± 105949)	3187000 ns (± 87416)	1.02
add-zero-reuse-first/create	1561500 ns (± 72468)	1582500 ns (± 67037)	0.99
add-zero-reuse-first/rewrite	9500 ns (± 1918)	10000 ns (± 2236)	0.95
add-zero-lots-of-reuse-first/create	1551000 ns (± 20876)	1578000 ns (± 55869)	0.98
add-zero-lots-of-reuse-first/rewrite	754000 ns (± 5431)	775000 ns (± 15515)	0.97

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