Fix incorrect usage of TypeVar#inst

TypeVar#isInstantiated uses instanceOpt, which extends inst.  So either
use the pair .inst/.inst.exists or isInstantiated/.instanceOpt.  The way
it was made the TypeVar in tests/fuzzy/i9293.scala return NoType with
this call.  Looking around, looks like the usage of .inst directly is
much more rare and nowhere within TypeComparer (or its parents) so I
opted for instanceOpt.

Author: dwijnand

Diff for: compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1596,7 +1596,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         val tycon1 = liftToThis(tp.tycon)
         if (tycon1 ne tp.tycon) tp.derivedAppliedType(tycon1, tp.args) else tp
       case tp: TypeVar if tp.isInstantiated =>
-        liftToThis(tp.inst)
+        liftToThis(tp.instanceOpt)
       case tp: AnnotatedType =>
         val parent1 = liftToThis(tp.parent)
         if (parent1 ne tp.parent) tp.derivedAnnotatedType(parent1, tp.annot) else tp