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

In preparation for supporting vector expansion.

Also drop a variant of ExpandLibCall, of which the MULO expansions
were the only user.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353611 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed