[Flang][OpenMP] Fix conversion of nested loops for SIMD directive

Flang was not able to convert simd directive which contains nested
Fortran loops. The nested Fortran loops inside SIMD directive
are modelled as FIR loops and they need to be translated into LLVM
MLIR dialect.

Differential Revision: https://reviews.llvm.org/D131402



Reviewed by: peixin

Signed-off-by: Dominik Adamski <dominik.adamski@amd.com>