Implement a poor man's jump table.

We will eventually support real jump tables, but for now just expand
br_table into a sequence of conditional branches.

lib/cretonne/meta/base/legalize.py

@@ -53,6 +53,7 @@ expand.custom_legalize(insts.heap_addr, 'expand_heap_addr')
 # TODO: Add sufficient XForm syntax that we don't need to hand-code these.
 expand.custom_legalize(insts.trapz, 'expand_cond_trap')
 expand.custom_legalize(insts.trapnz, 'expand_cond_trap')
+expand.custom_legalize(insts.br_table, 'expand_br_table')
 
 # Custom expansions for floating point constants.
 # These expansions require bit-casting or creating constant pool entries.