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/codegen/src/regalloc/branch_splitting.rs

@@ -2,8 +2,6 @@
 //!
 //! One of the reason for splitting edges is to be able to insert `copy` and `regmove` instructions
 //! between a conditional branch and the following terminator.
-#![cfg(feature = "basic-blocks")]
-
 use alloc::vec::Vec;
 
 use crate::cursor::{Cursor, EncCursor};