Change ConstantFoldInstOperands to take Instruction instead of opcode and type. NFC.
The previous form, taking opcode and type, is moved to an internal
helper and the new form, taking an instruction, is a wrapper around this
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.
Subscribers: dblaikie, llvm-commits
Differential Revision: http://reviews.llvm.org/D16383