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.

llvm-svn: 244168