[BPF] Add a few new insns under cpu=v4

In [1], a few new insns are proposed to expand BPF ISA to
  . fixing the limitation of existing insn (e.g., 16bit jmp offset)
  . adding new insns which may improve code quality
    (sign_ext_ld, sign_ext_mov, st)
  . feature complete (sdiv, smod)
  . better user experience (bswap)

This patch implemented insn encoding for
  . sign-extended load
  . sign-extended mov
  . sdiv/smod
  . bswap insns
  . unconditional jump with 32bit offset

The new bswap insns are generated under cpu=v4 for __builtin_bswap.
For cpu=v3 or earlier, for __builtin_bswap, be or le insns are generated
which is not intuitive for the user.

To support 32-bit branch offset, a 32-bit ja (JMPL) insn is implemented.
For conditional branch which is beyond 16-bit offset, llvm will do
some transformation 'cond_jmp' -> 'cond_jmp + jmpl' to simulate 32bit
conditional jmp. See BPFMIPeephole.cpp for details. The algorithm is
hueristic based. I have tested bpf selftest pyperf600 with unroll account
600 which can indeed generate 32-bit jump insn, e.g.,
        13:       06 00 00 00 9b cd 00 00 gotol +0xcd9b <LBB0_6619>

Eduard is working on to add 'st' insn to cpu=v4.

A list of llc flags:
  disable-ldsx, disable-movsx, disable-bswap,
  disable-sdiv-smod, disable-gotol
can be used to disable a particular insn for cpu v4.
For example, user can do:
  llc -march=bpf -mcpu=v4 -disable-movsx t.ll
to enable cpu v4 without movsx insns.

References:
  [1] https://lore.kernel.org/bpf/4bfe98be-5333-1c7e-2f6d-42486c8ec039@meta.com/

Differential Revision: https://reviews.llvm.org/D144829