Pick argument and return types based on opcode constraints (#5947)

* Pick argument and return types based on opcode constraints

Co-authored-by: Jamey Sharp <jsharp@fastly.com>

* Lazily build the OPCODE_SIGNATURES list

* Skip unsupported isplit/iconcat cases

* Add an issue reference for the isplit/iconcat exemption

* Refactor the deny lists to use exceptions!, and remove redundant entries

---------

Co-authored-by: Jamey Sharp <jsharp@fastly.com>

cranelift/fuzzgen/Cargo.toml

@@ -16,4 +16,5 @@ cranelift-native = { workspace = true }
 
 anyhow = { workspace = true }
 arbitrary = "1.0.0"
+once_cell = { workspace = true }
 target-lexicon = { workspace = true, features = ["std"] }