[SelectionDAG] Merge the two identical ExpandChainLibCall methods from...

[SelectionDAG] Merge the two identical ExpandChainLibCall methods from LegalizeTypes and LegalizeDAG to one version in TaretLowering.

Reviewers: RKSimon, efriedma, spatel

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70354