[InstCombine] fold not-shift of signbit to icmp+zext

https://alive2.llvm.org/ce/z/j_8Wz9

The arithmetic shift was converted to logical shift with:
24607860

That does not seem to uncover any other missing/conflicting folds,
so convert directly to signbit test + cast.

We still need to fold the pattern with logical shift to test + cast.

This allows reducing patterns where the output type is not
the same as the input value:
https://alive2.llvm.org/ce/z/nydwFV

Fixes #57394