add intrinsics_gen dependence on LLVMPollyLib

to avoid build errors like this:
In file included from ../include/llvm/IR/IntrinsicInst.h:30:0,
                 from ../tools/polly/lib/CodeGen/BlockGenerators.cpp:28:
../include/llvm/IR/Intrinsics.h:41:34: fatal error: llvm/IR/Intrinsics.gen: No such file or directory