[X86] Add test case to show failure to combine away negates that may be created by...

[X86] Add test case to show failure to combine away negates that may be created by mul by constant expansion.

Mul by constant can expand to a sequence that ends with a negate. If the next instruction is an add or sub we might be able to fold the negate away.

We currently fail to do this because we explicitly don't add anything to the DAG combine worklist when we expand multiplies. This is primarily to keep the multipy from being reformed, but we should consider adding the users to worklist.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337843 91177308-0d34-0410-b5e6-96231b3b80d8