[PredicateInfo] Add additional RenamedOp field to PB.

 OriginalOp of a predicate always refers to the original IR
value that was renamed. So for nested predicates of the same value, it
will always refer to the original IR value.

For the use in SCCP however, we need to find the renamed value that is
currently used in the condition associated with the predicate. This
patch adds a new RenamedOp field to do exactly that.

NewGVN currently relies on the existing behavior to merge instruction
metadata. A test case to check for exactly that has been added in
195fa4bf.

Reviewers: efriedma, davide, nikic

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D78133