[SelectionDAG] Extract [US]MULO expansion into TL method; NFC

In preparation for supporting vector expansion.

Add an isPostTypeLegalization flag to makeLibCall(), because this
expansion relies on the legalized form using MERGE_VALUES. Drop
the corresponding variant of ExpandLibCall, which is no longer used.

Differential Revision: https://reviews.llvm.org/D58006

llvm-svn: 354226