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/frontend/src/frontend.rs

@@ -480,7 +480,6 @@ impl<'a> FunctionBuilder<'a> {
         );
 
         // In debug mode, check that all blocks are valid basic blocks.
-        #[cfg(feature = "basic-blocks")]
         #[cfg(debug_assertions)]
         {
             // Iterate manually to provide more helpful error messages.