bpf, riscv: Optimize BPF_JSET BPF_K using andi on RV64

This patch optimizes BPF_JSET BPF_K by using a RISC-V andi instruction
when the BPF immediate fits in 12 bits, instead of first loading the
immediate to a temporary register.

Examples of generated code with and without this optimization:

BPF_JMP_IMM(BPF_JSET, R1, 2, 1) without optimization:

  20: li    t1,2
  24: and   t1,a0,t1
  28: bnez  t1,0x30

BPF_JMP_IMM(BPF_JSET, R1, 2, 1) with optimization:

  20: andi  t1,a0,2
  24: bnez  t1,0x2c

BPF_JMP32_IMM(BPF_JSET, R1, 2, 1) without optimization:

  20: li    t1,2
  24: mv    t2,a0
  28: slli  t2,t2,0x20
  2c: srli  t2,t2,0x20
  30: slli  t1,t1,0x20
  34: srli  t1,t1,0x20
  38: and   t1,t2,t1
  3c: bnez  t1,0x44

BPF_JMP32_IMM(BPF_JSET, R1, 2, 1) with optimization:

  20: andi  t1,a0,2
  24: bnez  t1,0x2c

In these examples, because the upper 32 bits of the sign-extended
immediate are 0, BPF_JMP BPF_JSET and BPF_JMP32 BPF_JSET are equivalent
and therefore the JIT produces identical code for them.

Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Björn Töpel <bjorn.topel@gmail.com>
Acked-by: Björn Töpel <bjorn.topel@gmail.com>
Link: https://lore.kernel.org/bpf/20200506000320.28965-5-luke.r.nels@gmail.com

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