[BPF] Teach DAG2DAG AND elimination about load intrinsics

As commented on the existing code:

  // The Reg operand should be a virtual register, which is defined
  // outside the current basic block. DAG combiner has done a pretty
  // good job in removing truncating inside a single basic block.

However, when the Reg operand comes from bpf_load_[byte | half | word]
intrinsics, the generic optimizer doesn't understand their results are
zero extended, so these single basic block elimination opportunities were
missed.

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
llvm-svn: 322534