Drop 'basic-blocks' feature (#1363)

* All: Drop 'basic-blocks' feature

This makes it so that 'basic-blocks' cannot be disabled and we can
start assuming it everywhere.

* Tests: Replace non-bb filetests with bb version

* Tests: Adapt solver-fixedconflict filetests to use basic blocks

cranelift/filetests/Cargo.toml

@@ -22,6 +22,3 @@ memmap = "0.7.0"
 num_cpus = "1.8.0"
 region = "2.1.2"
 byteorder = { version = "1.3.2", default-features = false }
-
-[features]
-basic-blocks = []