[X86] Qualify one of the heuristics in combineMul to only apply to positive multiply amounts.

This seems to slightly help the performance of one of our internal benchmarks. We probably need better heuristics here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339406 91177308-0d34-0410-b5e6-96231b3b80d8