[TargetLowering] add tests to show effect of setcc sub->shift; NFC

[TargetLowering] add tests to show effect of setcc sub->shift; NFC

There's effectively no difference for the cases with variables.
We just trade a sub for an add on those. But the case with a
subtract from constant would require an extra move instruction
on x86, so this looks like a reasonable generic combine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353619 91177308-0d34-0410-b5e6-96231b3b80d8