Add entity references as a new operand kind.

Define known entities in the cretonne.entities module.

meta/cretonne/entities.py

@@ -0,0 +1,23 @@
+"""
+The `cretonne.entities` module predefines all the Cretonne entity reference
+operand types. Thee are corresponding definitions in the `cretonne.entities`
+Rust module.
+"""
+
+from . import EntityRefKind
+
+
+#: A reference to an extended basic block in the same function.
+#: This is primarliy used in control flow instructions.
+ebb = EntityRefKind('ebb', 'An extended basic block in the same function.')
+
+#: A reference to a stack slot declared in the function preamble.
+stack_slot = EntityRefKind('stack_slot', 'A stack slot.')
+
+#: A reference to a function sugnature declared in the function preamble.
+#: Tbis is used to provide the call signature in an indirect call instruction.
+signature = EntityRefKind('signature', 'A function signature.')
+
+#: A reference to an external function declared in the function preamble.
+#: This is used to provide the callee and signature in a call instruction.
+function = EntityRefKind('function', 'An external function.')