gallivm: abort when trying to use non-existing intrinsic

gallivm: abort when trying to use non-existing intrinsic

Whenever llvm removes an intrinsic (we're using), we're hitting segfaults
due to llvm doing calls to address 0 in the jitted code instead.
However, Jose figured out we can actually detect this with
LLVMGetIntrinsicID(), so use this to abort, so we don't have to wonder
what got broken. (Of course, someone still needs to fix the code to
no longer use this intrinsic.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>