[InstCombine] peek through FP casts for sign-bit compares (PR36682)

This pattern came up in PR36682:
https://bugs.llvm.org/show_bug.cgi?id=36682
https://godbolt.org/g/LhuD9A

Equality checks are planned as a follow-up enhancement.

Differential Revision: https://reviews.llvm.org/D44367

llvm-svn: 328426