Mark spill and fill as can_store and can_load.

This allows GVN to avoid hoisting them. These will be to coarse for
things that want more precise dependence information, however we can
work that out when we build such things.

lib/cretonne/meta/base/instructions.py

@@ -501,7 +501,7 @@ spill = Instruction(
         This instruction behaves exactly like :inst:`copy`, but the result
         value is assigned to a spill slot.
         """,
-        ins=x, outs=a)
+        ins=x, outs=a, can_store=True)
 
 fill = Instruction(
         'fill', r"""
@@ -510,7 +510,7 @@ fill = Instruction(
         This instruction behaves exactly like :inst:`copy`, but creates a new
         SSA value for the spilled input value.
         """,
-        ins=x, outs=a)
+        ins=x, outs=a, can_load=True)
 
 src = Operand('src', regunit)
 dst = Operand('dst', regunit)