Instead of blindly looking past constantexpr casts, actually constant

fold them.  This correctly truncates constants that are too large for the
destination slot and makes the code easier to understand.  This fixes PR853
and Regression/CodeGen/X86/2006-07-28-AsmPrint-Long-As-Pointer.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29408 91177308-0d34-0410-b5e6-96231b3b80d8