* QuickMetadata has been expanded to get a `MDString` or a `MDTuple`
  from a `StringRef`.
* Introducing `skipCasts`, which, given a `Value`, returns the innermost
  part of the expression, skipping over casts.
* Introducing `isCallTo`, which, given an `Instruction`, returns whether
  it's a call to a specific function or not. `getCallTo` is a sister
  function to be used in `if` statements.
* Moving `skip` and `erase_if` in `ir-helpers.h` so that all the
  translation unit can benefit from their usage.