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/filetests/isa/x86/legalize-br-table-bb.clif

@@ -1,32 +0,0 @@
-test compile
-set opt_level=speed_and_size
-target x86_64
-feature "basic-blocks"
-; regex: V=v\d+
-; regex: EBB=ebb\d+
-
-function u0:0(i64) system_v {
-    ss0 = explicit_slot 1
-    jt0 = jump_table [ebb1]
-
-ebb0(v0: i64):
-    v1 = stack_addr.i64 ss0
-    v2 = load.i8 v1
-    br_table v2, ebb2, jt0
-; check:     $(oob=$V) = ifcmp_imm $(idx=$V), 1
-; ebb2 is replaced by ebb1 by fold_redundant_jump
-; nextln:    brif uge $oob, ebb1
-; nextln:    fallthrough $(inb=$EBB)
-; check:   $inb:
-; nextln:    $(final_idx=$V) = uextend.i64 $idx
-; nextln:    $(base=$V) = jump_table_base.i64 jt0
-; nextln:    $(rel_addr=$V) = jump_table_entry $final_idx, $base, 4, jt0
-; nextln:    $(addr=$V) = iadd $base, $rel_addr
-; nextln:    indirect_jump_table_br $addr, jt0
-
-ebb2:
-    jump ebb1
-
-ebb1:
-    return
-}

cranelift/filetests/filetests/isa/x86/legalize-br-table.clif

@@ -1,7 +1,6 @@
 test compile
 set opt_level=speed_and_size
 target x86_64
-feature !"basic-blocks"
 ; regex: V=v\d+
 ; regex: EBB=ebb\d+
 
@@ -14,7 +13,8 @@ ebb0(v0: i64):
     v2 = load.i8 v1
     br_table v2, ebb2, jt0
 ; check:     $(oob=$V) = ifcmp_imm $(idx=$V), 1
-; nextln:    brif uge $oob, ebb2
+; ebb2 is replaced by ebb1 by fold_redundant_jump
+; nextln:    brif uge $oob, ebb1
 ; nextln:    fallthrough $(inb=$EBB)
 ; check:   $inb:
 ; nextln:    $(final_idx=$V) = uextend.i64 $idx