[flang][NFC] Add select type test to check correct ordering

Somehow this test has been left behind in my sandbox. This
patch adds a lowering test for fir.select_type operation and
makes sure the dynamic type comaparison is done in the right
order when we have multiple CLASS IS type guard statement
for types that are linked.

This should have been posted with D138280.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D147807