Refactor some code, making it simpler.

Refactor some code, making it simpler.
When doing the initial pass of constant folding, if we get a constantexpr,
simplify the constant expr like we would do if the constant is folded in the
normal loop.

This fixes the missed-optimization regression in
Transforms/InstCombine/getelementptr.ll last night.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28224 91177308-0d34-0410-b5e6-96231b3b80d8