feat(riscv): Use the ! rewriter functions in the branch lowering

[tobiasgrosser]

Replace the proof-carrying WfRewriter calls in the branch instruction
selection with their panicking ! variants, which check their
preconditions at runtime instead. This removes all 24 sorrys from the
pass.

Dropping the sorrys re-enables the unused-variable linter for these
declarations, which exposes that convertBranch never uses its block
argument; remove it.

[github-actions]

VeIR Benchmarks

Details

Benchmark suite	Current: 30bf3b5	Previous: 86e3eaf	Ratio
add-fold-worklist/create	2138000 ns (± 89433)	2281000 ns (± 109561)	0.94
add-fold-worklist/rewrite	3955000 ns (± 60305)	3831500 ns (± 58442)	1.03
add-fold-worklist-local/create	2190500 ns (± 107057)	2402000 ns (± 116565)	0.91
add-fold-worklist-local/rewrite	3751000 ns (± 109623)	3722000 ns (± 84795)	1.01
add-zero-worklist/create	2166000 ns (± 68064)	2382000 ns (± 119840)	0.91
add-zero-worklist/rewrite	2544000 ns (± 117885)	2526000 ns (± 71001)	1.01
add-zero-reuse-worklist/create	1776000 ns (± 65563)	2149000 ns (± 79534)	0.83
add-zero-reuse-worklist/rewrite	2064000 ns (± 84022)	2033000 ns (± 48703)	1.02
mul-two-worklist/create	2136000 ns (± 75939)	2409000 ns (± 116407)	0.89
mul-two-worklist/rewrite	5594000 ns (± 121349)	5506500 ns (± 268762)	1.02
add-fold-forwards/create	2179000 ns (± 84253)	2325500 ns (± 115467)	0.94
add-fold-forwards/rewrite	2934000 ns (± 34602)	2939000 ns (± 55488)	1.00
add-zero-forwards/create	2092000 ns (± 80828)	2345500 ns (± 110446)	0.89
add-zero-forwards/rewrite	1937000 ns (± 38790)	1904500 ns (± 32208)	1.02
add-zero-reuse-forwards/create	1842000 ns (± 56403)	1946500 ns (± 98021)	0.95
add-zero-reuse-forwards/rewrite	1598000 ns (± 72220)	1562000 ns (± 43519)	1.02
mul-two-forwards/create	2138000 ns (± 44008)	2344000 ns (± 104495)	0.91
mul-two-forwards/rewrite	3548000 ns (± 31987)	3632000 ns (± 156014)	0.98
add-zero-reuse-first/create	1830000 ns (± 110467)	1993500 ns (± 107618)	0.92
add-zero-reuse-first/rewrite	9000 ns (± 3044)	8500 ns (± 3395)	1.06
add-zero-lots-of-reuse-first/create	1818000 ns (± 83812)	2041000 ns (± 98132)	0.89
add-zero-lots-of-reuse-first/rewrite	776000 ns (± 58875)	765000 ns (± 19967)	1.01

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

[tobiasgrosser]

I would appreciate a comment on the block argument removal. @regehr.