[InstCombine] refactor folds for mul with negated operands; NFCI

This keeps with our current usage of 'match' and is easier to see that
the optional NSW only applies in the non-constant operand case. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325140 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed