[InstCombine] form copysign from select of FP constants (PR44153)

This should be the last step needed to solve the problem in the
description of PR44153:
https://bugs.llvm.org/show_bug.cgi?id=44153

If we're casting an FP value to int, testing its signbit, and then
choosing between a value and its negated value, that's a
complicated way of saying "copysign":

(bitcast X) <  0 ? -TC :  TC --> copysign(TC,  X)

Differential Revision: https://reviews.llvm.org/D72643