[IR] `IRBuilderBase::CreateAdd()`: short-circuit `x + 0` --> `x`

There's precedent for that in `CreateOr()`/`CreateAnd()`.

The motivation here is to avoid bloating the run-time check's IR
in `SCEVExpander::generateOverflowCheck()`.

Refs. https://reviews.llvm.org/D109368#3089809