Fortran Wiki
LLVM Dragonegg

LLVM Dragonegg is a gcc plugin that replaces GCC’s optimizers and code generators with those from the LLVM project. It works with gcc-4.5 or newer, and “fully supports … Fortran”.

For this to work, you need the current version of the LLVM, the matching version of Dragonegg, which you will compile to create a library eg, which is then called from gfortran/gcc using -fplugin=./