Add a stack_check instruction.

This instruction loads a stack limit from a global variable and compares
it to the stack pointer, trapping if the stack has grown beyond the
limit.

Also add a expand_flags transform group containing legalization patterns
for ISAs with CPU flags.

Fixes #234.

lib/cretonne/meta/isa/intel/legalize.py

@@ -18,7 +18,7 @@ intel_expand = XFormGroup(
 
         Use Intel-specific instructions if needed.
         """,
-        isa=ISA, chain=shared.expand)
+        isa=ISA, chain=shared.expand_flags)
 
 a = Var('a')
 dead = Var('dead')