[LegalizeDAG] Convert strict fp nodes to libcalls without losing the chain.

Previously we mutated the node and then converted it to a libcall. But this loses the chain information.

This patch keeps the chain, but unfortunately breaks tail call optimization as the functions involved in deciding if a node is in tail call position can't handle the chain. But correct ordering seems more important to be right.

Somehow the SystemZ tests improved. I looked at one of them and it seemed that we're handling the split vector elements in a different order and that made the copies work better.

Differential Revision: https://reviews.llvm.org/D70334

Author: topperc