compiler: fix interface lowering miscompilation with reflect

When using reflect, arbitrary types can be synthesized. This invalidates
a few assumptions in the interface-lowering pass, that think they can
see all types that are in use in a program and optimize accordingly.

The file size impact depends on the specific program. Sometimes it's
nonexistent, sometimes it's rather hefty (up to 30% bigger). Especially
the samd21 targets seem to be affected, with a 2000-6000 bytes increase
in code size. A moderately large case (the stdlib test) increases by
4%/6%/15% depending on the target.

I hope that this increase could be mitigated, but I don't see an obvious
way to do that.

testdata/reflect.txt

@@ -335,3 +335,4 @@ float32 4 32
 float64 8 64
 complex64 8 64
 complex128 16 128
+type assertion succeeded for unreferenced type