[InstCombine] Use commutable matchers and m_OneUse in visitSub to shorten code. Add missing test cases.

In one case I removed commute handling for a multiply with a constant since we'll eventually get the constant on the right hand side.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299863 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed