[fir] Add fir.select_case conversion

The `fir.select_case` operation is converted to a if-then-else ladder.

Conversion of `fir.select_case` operation with character is not
implemented yet.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan, mehdi_amini

Differential Revision: https://reviews.llvm.org/D113484



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>