[NVPTX] Make sure we do not generate MULWIDE ISD nodes when optimizations are disabled

With optimizations disabled, we disable the isel patterns for mul.wide; but we
were still generating MULWIDE ISD nodes.  Now, we only try to generate MULWIDE
ISD nodes in DAGCombine if the optimization level is not zero.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213773 91177308-0d34-0410-b5e6-96231b3b80d8