Add operators to Register to supporting adding an offset to get
another Register.

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 getFirstNonPHI use the iterator-returning version.

This patch changes a bunch of call-sites calling getFirstNonPHI to use
getFirstNonPHIIt, which returns an iterator. All these call sites are
where it's obviously safe to fetch the iterator then dereference it. A
follow-up patch will contain less-obviously-safe changes.

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

---------

Co-authored-by: Stephen Tozer <Melamoto@gmail.com>

…on returning { i8, i128 }

Fixes https://github.com/llvm/llvm-project/issues/96432.

Identified with misc-include-cleaner.

Following the addition of the llvm.fake.use intrinsic and corresponding
MIR instruction, two further changes are planned: to add an
-fextend-lifetimes flag to Clang that emits these intrinsics, and to
have -Og enable this flag by default. Currently, some logic for handling
fake uses is gated by the optdebug attribute, which is intended to be
switched on by -fextend-lifetimes (and by extension -Og later on).
However, the decision was made that a general optdebug attribute should
be incompatible with other opt_ attributes (e.g. optsize, optnone),
since they all express different intents for how to optimize the
program. We would still like to allow -fextend-lifetimes with optsize
however (i.e. -Os -fextend-lifetimes should be legal), since it may be a
useful configuration and there is no technical reason to not allow it.

This patch resolves this by tracking MachineFunctions that have fake
uses, allowing us to run passes that interact with them and skip passes
that clash with them.

Now that basic blocks have numbers, we can replace the BB-to-MBB maps
and the visited set during ISel with vectors for faster lookup.

This is a preparation for changing the data structure of MBBMap.