[BACKEND][AMD] Add option to swap MIR (#8711)

We use TRITON_SWAP_MIR to specify the path to externally-provided MIR
file that can be used for compilation to ASM in LLVM backend, overriding
the one generated by regular pipeline.

This can be used to apply custom transformations on MIR and use it to
generate binary.

Author: tyb0807