[LoopIdiom] Introduce 'left-shift until bittest' idiom

The motivation here is the following inner loop in fp16/fp24 -> fp32 expander,
that runs as part of the floating-point DNG decompression in RawSpeed library:
https://github.com/darktable-org/rawspeed/blob/cd380bb9a209bd2e7a0e7022b0cab04528d151e7/src/librawspeed/decompressors/DeflateDecompressor.cpp#L112-L115
```
      while (!(fp32_fraction & (1 << 23))) {
        fp32_exponent -= 1;
        fp32_fraction <<= 1;
      }
```
(https://godbolt.org/z/r13YMh)
As one might notice, that loop is currently uncountable, and that whole code stays scalar.
Yet, it is rather trivial to make that loop countable:
 https://godbolt.org/z/do8WMz
and we can prove that via alive2:
 https://alive2.llvm.org/ce/z/7vQnji (ha nice, isn't it?)
... and that allow for the whole fp16->fp32 code to vectorize:
 https://godbolt.org/z/7hYr13

Now, while i'd love to get there, i feel like i should take it in steps.

For now, this introduces support for the most basic case,
where the bit position is known as a variable,
and the loop *will* go away (has no live-outs other than the recurrence,
no extra instructions in the loop).

I have added sufficient (i believe) test coverage,
and alive2 is happy with those transforms.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D91038

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