Make regalloc visit fallthrough_return instructions.

Add an explicit "is_ghost" property to selected instructions, and use
that to determine whether reload and coloring should visit instructions.
This allows them to visit fallthrough_return instructions and insert
fills and register moves as needed.

lib/codegen/meta-python/cdsl/instructions.py

@@ -92,6 +92,7 @@ class Instruction(object):
     :param is_indirect_branch: This is an indirect branch instruction.
     :param is_call: This is a call instruction.
     :param is_return: This is a return instruction.
+    :param is_ghost: This is a ghost instruction.
     :param can_trap: This instruction can trap.
     :param can_load: This instruction can load from memory.
     :param can_store: This instruction can store to memory.
@@ -108,6 +109,7 @@ class Instruction(object):
             'True for all indirect branch or jump instructions.',
             'is_call': 'Is this a call instruction?',
             'is_return': 'Is this a return instruction?',
+            'is_ghost': 'Is this a ghost instruction?',
             'can_load': 'Can this instruction read from memory?',
             'can_store': 'Can this instruction write to memory?',
             'can_trap': 'Can this instruction cause a trap?',