Driver support for optimisation passes #1501
Description
The current flang-new driver doesn't support optimisation.
When working on the SNAP perforamnce investigation, compiling the 2 files of hand-modified MLIR with clang -O3, gave 5-10% improvement, with no other changes.
The general framework for this is also required to allow specialized optimisation passes on FIR and LLVM-IR in Flang.