bpf: Tighten subregister definition check

The current subregister definition check stops after the MOV_32_64
instruction.

This means we are thinking all the following instruction sequences
are safe to be eliminated:

  MOV_32_64 rB, wA
  SLL_ri    rB, rB, 32
  SRL_ri    rB, rB, 32

However, this is *not* true. The source subregister wA of MOV_32_64 could
come from a implicit truncation of 64-bit register in which case the high
bits of the 64-bit register is not zeroed, therefore we can't eliminate
above sequence.

For example, for i32_val, we shouldn't do the elimination:

  long long bar ();

  int foo (int b, int c)
  {
    unsigned int i32_val = (unsigned int) bar();

    if (i32_val < 10)
      return b;
    else
      return c;
  }

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 327365