[flang][cuda] Support memory cleanup at a return statement

[khaki3]

We generate cuf.free and func.return twice if a return statement exists at the end of program.

program test
  integer, device :: a(10)
  return
end

% flang -x cuda test.cuf -mmlir --mlir-print-ir-after-all
error: loc("/path/to/test.cuf":3:3): 'func.return' op must be the last operation in the parent block
// -----// IR Dump After Fortran::lower::VerifierPass Failed () //----- //

Dumped IR:

  "func.func"() <{function_type = () -> (), sym_name = "_QQmain"}> ({
...
    "cuf.free"(%5#1) <{data_attr = #cuf.cuda<device>}> : (!fir.ref<!fir.array<10xi32>>) -> ()
    "func.return"() : () -> ()
    "cuf.free"(%5#1) <{data_attr = #cuf.cuda<device>}> : (!fir.ref<!fir.array<10xi32>>) -> ()
    "func.return"() : () -> ()
}
...

The routine genExitRoutine in Bridge.cpp is guarded by blockIsUnterminated() to make sure that func.return is generated only at the end of a block. However, we redundantly run bridge.fctCtx().finalizeAndKeep() before genExitRoutine in this case, resulting in two pairs of cuf.free and func.return. This PR fixes Bridge.cpp by using blockIsUnterminated() to guard finalizeAndKeep as well.