[mlir][LLVM] add `llvm.ssa.copy` intrinsic

This is quite the niche intrinsic, whose whole purpose is to be able to essentially split an SSA value to be able to attach additional information to the new value.
It therefore really acts like a noop that just passes through its argument. It interestingly does not have any support in CodeGen and is therefore required to also be deleted by any pass creating it.

Differential Revision: https://reviews.llvm.org/D155678