transmutability: uninit transition matches unit byte only

The previous implementation was inconsistent about transitions that
apply for an init byte. For example, when answering a query, an init
byte could use corresponding init transition. Init byte could also use
uninit transition, but only when the corresponding init transition was
absent. This behaviour was incompatible with DFA union construction.

Define an uninit transition to match an uninit byte only and update
implementation accordingly. To describe that `Tree::uninit` is valid
for any value, build an automaton that accepts any byte value.

Additionally, represent byte ranges uniformly as a pair of integers to
avoid special case for uninit byte.

Author: tmiasko

compiler/rustc_transmute/Cargo.toml

@@ -5,7 +5,6 @@ edition = "2024"
 
 [dependencies]
 # tidy-alphabetical-start
-itertools = "0.12"
 rustc_abi = { path = "../rustc_abi", optional = true }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_hir = { path = "../rustc_hir", optional = true }
@@ -15,6 +14,11 @@ smallvec = "1.8.1"
 tracing = "0.1"
 # tidy-alphabetical-end
 
+[dev-dependencies]
+# tidy-alphabetical-start
+itertools = "0.12"
+# tidy-alphabetical-end
+
 [features]
 rustc = [
     "dep:rustc_abi",