Compilation of LLVM can potentially be simplified

[giordano]

After JuliaLang/julia#50208 (which came about a time when Julia required LLVM 15, so probably not good for the current stage of this project), compilation of a fork of LLVM with Julia patches can theoretically be simplified, as long as there are no merge conflicts, by doing something like

LLVM_APPLY_JULIA_PATCHES = 1
LLVM_BASE_REF = sx-aurora-dev:...
LLVM_JULIA_REF = JuliaLang:julia-...

If the patch can be applied cleanly, this removes the need of keeping a branch around.

[giordano]

Actually, Valentin told me that we may be able to just use the VE target in upstream LLVM.