[LoopIdiom] 'left-shift until bittest' idiom: support rewriting loop as...

[LoopIdiom] 'left-shift until bittest' idiom: support rewriting loop as countable, allow extra cruft

The current state of the transform is still not enough to support
my motivational pattern, because it has one more "induction variable".

I have delayed posting this patch, because originally even just rewriting
the loop as countable wasn't enough to nicely transform my motivational pattern,
because i expected that extra IV to be rewritten afterwards,
but it wasn't happening until i fixed that in D91800.

So, this patch allows the  'left-shift until bittest' loop idiom
as long as the inserted ops are cheap,
and lifts any and all extra use checks on the instructions.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D92754