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/base/instructions.py

@@ -844,7 +844,7 @@ vsplit = Instruction(
         the lanes from ``x``. The result may be two scalars if ``x`` only had
         two lanes.
         """,
-        ins=x, outs=(lo, hi))
+        ins=x, outs=(lo, hi), is_ghost=True)
 
 Any128 = TypeVar(
         'Any128', 'Any scalar or vector type with as most 128 lanes',
@@ -864,7 +864,7 @@ vconcat = Instruction(
 
         It is possible to form a vector by concatenating two scalars.
         """,
-        ins=(x, y), outs=a)
+        ins=(x, y), outs=a, is_ghost=True)
 
 c = Operand('c', TxN.as_bool(), doc='Controlling vector')
 x = Operand('x', TxN, doc='Value to use where `c` is true')
@@ -2009,7 +2009,7 @@ isplit = Instruction(
         Returns the low half of `x` and the high half of `x` as two independent
         values.
         """,
-        ins=x, outs=(lo, hi))
+        ins=x, outs=(lo, hi), is_ghost=True)
 
 
 NarrowInt = TypeVar(
@@ -2029,6 +2029,6 @@ iconcat = Instruction(
         the same number of lanes as the inputs, but the lanes are twice the
         size.
         """,
-        ins=(lo, hi), outs=a)
+        ins=(lo, hi), outs=a, is_ghost=True)
 
 GROUP.close()

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?',