[CodeGen] Pull MMI helpers from FunctionLoweringInfo to MMI. NFC.

They're not SelectionDAG- or FunctionLoweringInfo-specific.  They
are, however, specific to building MMI from IR.
We could make them members, but it's nice having MMI be a "simple" data
structure and this logic kept separate.

This also lets us reuse them from GlobalISel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287167 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed