[vm/aot] Fix handling of dynamic calls through getters

When discovering getters which might be potentially hit by a dynamic
call through a getter we need to ignore method extractors.

DispatchTableGenerator eagerly creates some of the method extractors
and these extractors were accidentally pulled in by
CheckForNewDynamicFunctions(): if there is an dynamic invocation
of `foo` in the program we would compile all getters `get:foo` 
because `x.foo()` might be a dynamic call-through-getter `x.foo.call()`.

However this logic should not apply to method extractors because
`x.foo(...)` would never hit an extractor for `foo`, it would
hit method `foo` itself.

Removes 15K (0.04%) of total uncompressed code size on a large 
internal application.

Issue https://github.com/dart-lang/sdk/issues/41647

Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-linux-release-x64-try,vm-kernel-precomp-nnbd-linux-release-x64-try
Change-Id: I46e5f06fe4a3ee6efe61ed0f36675837a728e031
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152804
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>