[bpf] add support for bpf pseudo instruction

[bpf] add support for bpf pseudo instruction

Expose bpf pseudo load instruction via intrinsic. It is used by front-ends that
can encode file descriptors directly into IR instead of relying on relocations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233396 91177308-0d34-0410-b5e6-96231b3b80d8