[flang] Fix llvm-test-suite/Fortran/gfortran/torture/execute/st_function_1.f90

I just broke the test llvm-test-suite/Fortran/gfortran/torture/execute/st_function_1.f90
with a recent patch.  The bug was obvious, as is the fix, which works, so
I'm just pushing it directly to make the build bots happy.