[InstCombine] add folds for xor-of-icmp signbit tests (PR36682)

This is a retry of r328119 which was reverted at r328145 because
it could crash by trying to combine icmps with different operand
types. This version has a check for that and additional tests.

Original commit message:

This is part of solving:
https://bugs.llvm.org/show_bug.cgi?id=36682

There's also a leftover improvement from the long-ago-closed:
https://bugs.llvm.org/show_bug.cgi?id=5438

https://rise4fun.com/Alive/dC1

llvm-svn: 328197