[IPSCCP] Use input operand instead of OriginalOp for ssa_copy.

OriginalOp of a Predicate refers to the original IR value,
before renaming. While solving in IPSCCP, we have to use
the operand of the ssa_copy instead, to avoid missing
updates for nested conditions on the same IR value.

Fixes PR39772.

llvm-svn: 347524