ValueMapper: Simplify remap() helper function, NFC
ValueMapper: Simplify remap() helper function, NFC

Rename `remap()` to `remapOperands()`, and restrict its contract to
remapping operands.  Previously, it also called `mapToMetadata()`, but
this logic is hard to reason about externally.  In particular, this
refactors `mapUniquedNode()` to avoid redundant mapping calls, taking
advantage of the RAUWs that are already in place.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@244168 91177308-0d34-0410-b5e6-96231b3b80d8

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