[LoopIdiom] 'left-shift until bittest' idiom: support canonical sign bit mask

If the bitmask is for sign bit, instcombine would have canonicalized
the pattern into a proper sign bit check. Supporting that is still
simple, but requires a bit of a roundtrip - we first have to use
`decomposeBitTestICmp()`, and the rest again just works.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D91726