bpf: check illegal usage of XADD insn return value

Currently, BPF has XADD (locked add) insn support and the
asm looks like:
  lock *(u32 *)(r1 + 0) += r2
  lock *(u64 *)(r1 + 0) += r2
The instruction itself does not have a return value.

At the source code level, users often use
  __sync_fetch_and_add()
which eventually translates to XADD. The return value of
__sync_fetch_and_add() is supposed to be the old value
in the xadd memory location. Since BPF::XADD insn does not
support such a return value, this patch added a PreEmit
phase to check such a usage. If such an illegal usage
pattern is detected, a fatal error will be reported like
  line 4: Invalid usage of the XADD return value
if compiled with -g, or
  Invalid usage of the XADD return value
if compiled without -g.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 342692