Introduce ConstantFoldCastOperand function and migrate some callers of ConstantFoldInstOperands to use it.  NFC.

Summary:
Although this is a slight cleanup on its own, the main motivation is to
refactor the constant folding API to ease migration to opaque pointers.
This will be follow-up work.

Reviewers: eddyb

Subscribers: zzheng, dblaikie, llvm-commits

Differential Revision: http://reviews.llvm.org/D16380

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258390 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed