As part of the "RemoveDIs" project, BasicBlock::iterator now carries a
debug-info bit that's needed when getFirstNonPHI and similar feed into
instruction insertion positions. Call-sites where that's necessary were
updated a year ago; but to ensure some type safety however, we'd like to
have all calls to moveBefore use iterators.

This patch adds a (guaranteed dereferenceable) iterator-taking
moveBefore, and changes a bunch of call-sites where it's obviously safe
to change to use it by just calling getIterator() on an instruction
pointer. A follow-up patch will contain less-obviously-safe changes.

We'll eventually deprecate and remove the instruction-pointer
insertBefore, but not before adding concise documentation of what
considerations are needed (very few).

Support `MachineFunction` in `OptNoneInstrumentation`, also add
`isRequired` to all necessary passes.

- Add `SlotIndexesAnalysis`.
- Add `SlotIndexesPrinterPass`.
- Use `SlotIndexesWrapperPass` in legacy pass.

The StackColoring pass creates two instances of BitVector per basic
block until the fixed point is reached.  Each instance may involve a
heap allocation depending on its size.

This patch declares them outside the loop, saving 1.40% of heap
allocations during the compilation of CGBuiltin.cpp.ii, a preprocessed
version of CGBuiltin.cpp.

This reverts commit 08b306dc8e7c0b2498f4f194a3c51686d56dbd20.

it causes the following assertion failure:
llvm/include/llvm/CodeGen/MachineFrameInfo.h:530: int64_t
llvm::MachineFrameInfo::getObjectOffset(int) const: Assertion
`!isDeadObjectIndex(ObjectIdx) && "Getting frame offset for a dead
object?"' failed.