[flang] Add fir.select_type conversion to if-then-else ladder

Convert fir.select_type operation to an if-then-else ladder.
The type guards are sorted before the conversion so it follows the
execution of SELECT TYPE construct as mentioned in 11.1.11.2 point 4
of the Fortran standard.

Depends on D138279

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D138280