* `replaceOp` replaces all uses of the original op and erases the old
op.
* `replaceAllUsesWith` replaces all uses of the original op/value/block.
It does not erase any IR.
This commit renames `replaceOpWithIf` to `replaceUsesWithIf`.
`replaceOpWithIf` was a misnomer because the function never erases the
original op. Similarly, `replaceOpWithinBlock` is renamed to
`replaceUsesWithinBlock`. (No "operation replaced" is sent because the
op is not erased.)
Also improve comments.
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