Add Automatic SSR Inference

[ThomasRupf]

General

For compilation flags, see readme.
Note that SSR inference uses the new pass manager, which is invoked with -fexperimental-new-pass-manager in Clang.

Files

Optimisation Pipeline

File	Purpose
llvm/lib/Transforms/SSR/SSRInferencePass.cpp (and header)	main pass (inserted into pipeline)
llvm/lib/Transforms/SSR/SSRGenerationPass.cpp (and header)	code generation pass
llvm/lib/Analysis/AffineAccessAnalysis.cpp (and header)	analysis that finds square affine access patterns

RISC-V Backend

All in llvm/lib/Target/RISC-V (no header files)

File	Purpose
RISCVExpandSSRInstsPostRegAlloc.cpp	expands push/pop pseudo instructions after reg-alloc

Not scheduled in pipeline

File	Purpose
RISCVSSRReassociate.cpp	experimental reassociation pass
RISCVSSRStatistics.cpp	count number of load/store push/pop per loop
Snitch/SNITCHAutoFrep.cpp	see "autofrep" paragraph in thesis