[InstSimplify] Expose generic interface for replaced operand simplification

Users, especially the Attributor, might replace multiple operands at
once. The actual implementation of simplifyWithOpReplaced is able to
handle that just fine, the interface was simply not allowing to replace
more than one operand at a time. This is exposing a more generic
interface without intended changes for existing code.

Differential Revision: https://reviews.llvm.org/D106189