Allow SMUL_LOHI and UMUL_LOHI to be narrow to MUL on targets where MUL is Custom rather than Legal.  Even if the target is doing some kind of expansion for MUL, it's pretty much guaranteed to be more efficent than whatever it does for SMUL_LOHI or UMUL_LOHI!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199678 91177308-0d34-0410-b5e6-96231b3b80d8