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()