[BPF] Fix a bug in BPFMISimplifyPatchable pass

LLVM BPF pass SimplifyPatchable is used to do necessary
code conversion for CO-RE operations. When studying bpf
selftest 'exhandler', I found a corner case not handled properly.
The following is the C code, modified from original 'exhandler'
code.
  int g;
  int test(struct t1 *p) {
    struct t2 *q = p->q;
    if (q)
      return 0;
    struct t3 *f = q->f;
    if (!f) g = 5;
    return 0;
  }

For code:
  struct t3 *f = q->f;
  if (!f) ...
The IR before BPFMISimplifyPatchable pass looks like:
  %5:gpr = LD_imm64 @"llvm.t2:0:8$0:1"
  %6:gpr = LDD killed %5:gpr, 0
  %7:gpr = LDD killed %6:gpr, 0
  JNE_ri killed %7:gpr, 0, %bb.3
  JMP %bb.2
Note that compiler knows q = 0 based dataflow and value analysis.
The correct generated code after the pass should be
  %5:gpr = LD_imm64 @"llvm.t2:0:8$0:1"
  %7:gpr = LDD killed %5:gpr, 0
  JNE_ri killed %7:gpr, 0, %bb.3
  JMP %bb.2

But the current implementation did further optimization for the
above code and generates
  %5:gpr = LD_imm64 @"llvm.t2:0:8$0:1"
  JNE_ri killed %5:gpr, 0, %bb.3
  JMP %bb.2
which is incorrect.

This patch added a cache to remember those load insns not associated
with CO-RE offset value and will skip these load insns during
transformation.

Differential Revision: https://reviews.llvm.org/D123883