[SeparateConstOffsetFromGEP] Fix: `b - a` matched `a - b` during reuniteExts

During the SeparateConstOffsetFromGEP pass, a - b and b - a will be
considered equivalent in some instances.

An example- the IR contains:

  BB1:
      %add = add %a, 511
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %sub1 = sub %add, %b
      %gep = getelementptr float, ptr %p, %sub1

Step 1 in the SeparateConstOffsetFromGEP pass, after split constant index:

  BB1:
      %add = add %a, 511
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %sub.t = sub %a, %b
      %gep.base = getelementptr float, ptr %p, %sub.t
      %gep = getelementptr float, ptr %gep.base, 511

Step 2, after reuniteExts:

  BB1:
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %gep.base = getelementptr float, ptr %p, %sub2
      %gep = getelementptr float, ptr %gep.base, 511

Obviously, reuniteExts treated a - b and b - a as equivalent.
This patch fixes that.

Reviewed By: nikic, spatel

Differential Revision: https://reviews.llvm.org/D143542