Add a new `CreateIntrinsic` overload with no `Types`, useful for
creating calls to non-overloaded intrinsics that don't need additional
mangling.

Simplify iteration over the ToReplace vector, and some related cosmetic
cleanups.

We need to promote 8/16-bit cases to 32-bit. Unfortunately we are
missing demanded bits optimizations on readfirstlane, so we end up
emitting
an and instruction on the input. I'm also surprised this pass isn't
handling
half or bfloat yet.

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).

Use references instead of pointers for most state and common up some of
the initialization between the legacy and new pass manager paths.

Convert many instances of:
  Fn = Intrinsic::getOrInsertDeclaration(...);
  CreateCall(Fn, ...)
to the equivalent CreateIntrinsic call.

Rename the function to reflect its correct behavior and to be consistent
with `Module::getOrInsertFunction`. This is also in preparation of
adding a new `Intrinsic::getDeclaration` that will have behavior similar
to `Module::getFunction` (i.e, just lookup, no creation).

Use poison for an unused input to the permlanex16 intrinsic, to improve
register allocation and avoid an unnecessary v_mov instruction.