cloog_loop_merge: ensure loop domains are convex, i.e., not unions
cloog_loop_merge: ensure loop domains are convex, i.e., not unions

Most of the code, especially the clast construction, assumes
the loop domains are not unions.
cloog_loop_separate ensures this property in all cases,
but cloog_loop_merge would fail to ensure this property
if called on a list of a single loop.

Syllo/cloog,epowers/cloog,VanirLLVM/toolchain_cloog,epowers/cloog,Syllo/cloog,SaberMod/cloog-current,periscop/cloog,heysion/cloog-clone,VanirLLVM/toolchain_cloog,VanirLLVM/toolchain_cloog,heysion/cloog-clone,UBERTC/cloog,cfx-next/toolchain_cloog-upstream,cfx-next/toolchain_cloog-upstream,heysion/cloog-clone,evaautomation/cloog,cfx-next/toolchain_cloog-upstream,Distrotech/cloog,epowers/cloog,Distrotech/cloog,periscop/cloog,Distrotech/cloog,SaberMod/cloog-current,evaautomation/cloog,UBERTC/cloog,UBERTC/cloog