[mlir][cse] do not replace operands in previously simplified operations

If an operation has been inserted as a key in to the known values
hashtable, then it can not be modified in a way which changes its hash.
This change avoids modifying the operands of any previously recorded
operation, which prevents their hash from changing.

In an SSACFG region, it is impossible to visit an operation before
visiting its operands, so this is not a problem. This situation can only
happen in regions without strict dominance, such as graph regions.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D99486