Issue 8262 -  ICE(mtype.c) alias this to alias of an expression tuple

https://issues.dlang.org/show_bug.cgi?id=8262

This PR doesn't change anything about what should happen with a tuple being aliased to "this", but merely avoids the crash. The net result seems to be that implicite conversions don't take place.

This is blocking https://github.com/D-Programming-Language/druntime/pull/1057 ATM.