IR: Rewrite ConstantUniqueMap

Rewrite `ConstantUniqueMap` to be more similar to
`ConstantAggrUniqueMap`.

  - Use a `DenseMap` with custom MapInfo instead of a `std::map` with
    linear lookups and deletion.
  - Don't waste memory explicitly storing (heavyweight) keys.

Only `ConstantExpr` and `InlineAsm` actually use this data structure, so
I also updated them to use it.

This code cleanup is a precursor to reducing RAUW traffic on
`ConstantExpr` -- I felt badly adding a new (linear) call to
`ConstantUniqueMap::FindExistingKey`, so this designs away the concern.

A follow-up commit will transition the users of `ConstantAggrUniqueMap`
over.