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.

cranelift/filetests/regalloc/fallthrough-return.clif

@@ -0,0 +1,23 @@
+test regalloc
+target x86_64
+
+; Test that fallthrough returns are visited by reload and coloring.
+
+function %foo() -> f64 {
+  fn0 = %bar()
+
+ebb0:
+  v0 = f64const 0.0
+  call fn0()
+  fallthrough_return v0
+}
+; check: fill v0
+
+function %foo() -> f64 {
+  fn0 = %bar() -> f64, f64
+
+ebb0:
+  v0, v1 = call fn0()
+  fallthrough_return v1
+}
+; check: regmove v1, %xmm1 -> %xmm0