[EarlyCSE] Retain poison flags, if program is UB if poison.

Poison-generating flags can be retained during CSE on the earlier
instruction , *if* the earlier instruction being poison causes UB. For
now, always take AND for floating point instructions.

https://alive2.llvm.org/ce/z/4K3D7P

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D115247