-optimise changes the position of some pattern matching errors

[smarter]

This affects everything in https://github.com/lampepfl/dotty/tree/master/tests/neg-no-optimise:

% dotc tests/neg-no-optimise/patmat1.scala
-- Error: tests/neg-no-optimise/patmat1.scala:21:11 ----------------------------
21 |      case D() =>  // error: neither a subtype not a supertype
   |           ^
   |        this case is unreachable since class C and class D are unrelated
one error found

% dotc -optimise tests/neg-no-optimise/patmat1.scala
-- Error: tests/neg-no-optimise/patmat1.scala:18:4 -----------------------------
18 |    cc match {
   |    ^^
   |    this case is unreachable since class C and class D are unrelated
one error found

[allanrenucci]

Closing as we removed the optimisations in #4799