SelectionDAG: Factor ISD::MUL lowering code out of DAGTypeLegalizer

This code has been moved to a new function in the TargetLowering
class called expandMUL().  The purpose of this is to be able
to share lowering code between the SelectionDAGLegalize and
DAGTypeLegalizer classes.

No functionality changed intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206036 91177308-0d34-0410-b5e6-96231b3b80d8
SelectionDAG: Use helper function to improve legalization of ISD::MUL

The TargetLowering::expandMUL() helper contains lowering code extracted
from the DAGTypeLegalizer and allows the SelectionDAGLegalizer to expand more
ISD::MUL patterns without having to use a library call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206037 91177308-0d34-0410-b5e6-96231b3b80d8