[InstCombine] reduce compare of signbits of 2 values

Test if 2 values have different or same signbits:
(X u>> BitWidth - 1) == zext (Y s> -1) --> (X ^ Y) < 0
(X u>> BitWidth - 1) != zext (Y s> -1) --> (X ^ Y) > -1

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

As noted in #60242, these patterns regressed between the
14.0 and 15.0 releases - probably due to a change in
canonicalization of related patterns.

The related patterns for testing if 2 values are both
pos/neg appear to be handled already.