Remove support for llvm 3.1

The LLVM 3.1 support removed. Cleaned up dead code with LLVM_3_0. Reversed #ifdefs from (LLVM_3_4 || LLVM_3_5 || LLVM_3_6) to (!LLVM_3_2 && !LLVM_3_3). The changes are tested 02.08.14 with ISPC nightly testing system.