[InstCombine] Clear sign-bit of the constant magnitude in copysign (#85787)

Alive2: https://alive2.llvm.org/ce/z/vFykcZ
Address the comment
https://github.com/llvm/llvm-project/pull/85772#discussion_r1530179048.

Unfortunately, non-splat vector constants are not supported because we
haven't implemented constant folding of fabs with vector operands.