BPF: fix a CORE optimization bug

For the test case in this patch like below
  struct t { int a; } __attribute__((preserve_access_index));
  int foo(void *);
  int test(struct t *arg) {
      long param[1];
      param[0] = (long)&arg->a;
      return foo(param);
  }

The IR right before BPF SimplifyPatchable phase:
  %1:gpr = LD_imm64 @"llvm.t:0:0$0:0"
  %2:gpr = LDD killed %1:gpr, 0
  %3:gpr = ADD_rr %0:gpr(tied-def 0), killed %2:gpr
  STD killed %3:gpr, %stack.0.param, 0
After SimplifyPatchable phase, the incorrect IR is generated:
  %1:gpr = LD_imm64 @"llvm.t:0:0$0:0"
  %3:gpr = ADD_rr %0:gpr(tied-def 0), killed %1:gpr
  CORE_MEM killed %3:gpr, 306, %0:gpr, @"llvm.t:0:0$0:0"

Note that CORE_MEM pseudo op is introduced to encode
memory operations related to CORE. In the above, we intend
to check whether we have a store like
   *(%3:gpr + 0) = ...
and if this is the case, we could replace it with
   *(%0:gpr + @"llvm.t:0:0$0:0"_ = ...

Unfortunately, in the above, IR for the store is
   *(%stack.0.param + 0) = %3:gpr
and transformation should not happen.

Note that we won't have problem if the actual CORE
dereference (arg->a) happens.

This patch fixed the problem by skip CORE optimization if
the use of ADD_rr result is not the base address of the store
operation.

Differential Revision: https://reviews.llvm.org/D78466