radeonsi: Allow dumping LLVM IR before optimization passes

radeonsi: Allow dumping LLVM IR before optimization passes

Set R600_DEBUG=preoptir to dump the LLVM IR before optimization passes,
to allow diagnosing problems caused by optimization passes.

Note that in order to compile the resulting IR with llc, you will first
have to run at least the mem2reg pass, e.g.

opt -mem2reg -S < shader.ll | llc -march=amdgcn -mcpu=bonaire

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com> (original patch)
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (w/ debug flag)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>