[InferAddressSpaces] Enhance the handling of cosntexpr.

Summary:
- Constant expressions may not be added in strict postorder as the
  forward instruction scan order. Thus, for a constant express (CE0), if
  its operand (CE1) is used in an previous instruction, they are not in
  postorder. However, different from
  `cloneInstructionWithNewAddressSpace`,
  `cloneConstantExprWithNewAddressSpace` doesn't bookkeep uninferred
  instructions for later resolving. That results in failure of inferring
  constant address.
- This patch adds the support to infer constant expression operand
  recursively, since there won't be loop, if that operand is another
  constant expression.

Reviewers: arsenm

Subscribers: jholewinski, jvesely, wdng, nhaehnle, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61760

llvm-svn: 360431