[flang] Switch select-case-statement.f90 to new lowering (llvm#168754)

test/Lower/select-case-statement.f90 was still using the old lowering.
Modified the test with FIR generated using the new lowering. Changed the
test to use flang_fc1 instead of bbc and added testing for -O0 and -O1,
since character comparison lowering is done differently at -O0 (uses
runtime function) and -O1 (inlines some cases). Use different FileCheck
prefixes for different optimization levels (CHECK-O0 for -O0, CHECK-O1
for -O1, CHECK for both).

Author: eugeneepshteyn