Legalize br_table to a BB-like format.

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

@@ -1,6 +1,8 @@
 test compile
 
 target x86_64
+; regex: V=v\d+
+; regex: EBB=ebb\d+
 
 function u0:0(i64) system_v {
     ss0 = explicit_slot 1
@@ -10,6 +12,14 @@ 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
+; nextln:    brif uge $oob, ebb2
+; nextln:    fallthrough $(inb=$EBB)
+; check:   $inb:
+; nextln:    $(base=$V) = jump_table_base.i64 jt0
+; nextln:    $(rel_addr=$V) = jump_table_entry.i64 $idx, $base, 4, jt0
+; nextln:    $(addr=$V) = iadd $base, $rel_addr
+; nextln:    indirect_jump_table_br $addr, jt0
 
 ebb2:
     jump ebb1