Enable basic block checks through a feature. (#856)

This allows prefixing BB-specific code with "#[cfg(feature = "basic-blocks")]",
which avoids having to reference an environment variable across the codebase.

The easiest way to enable the feature locally is to add the arguments
'features = ["basic-blocks"]' to the workspace Cargo.toml, where it defines
the cranelift-codegen dependency.

cranelift/codegen/Cargo.toml

@@ -62,6 +62,9 @@ riscv = []
 # For dependent crates that want to serialize some parts of cranelift
 enable-serde = ["serde"]
 
+# Temporary feature that enforces basic block semantics.
+basic-blocks = []
+
 [badges]
 maintenance = { status = "experimental" }
 travis-ci = { repository = "CraneStation/cranelift" }