[SCEV] ScalarEvolution::createSCEV(): clarify no-wrap flag propagation for shift by bitwidth-1

Summary:
There was this comment here previously:
```
-        // It is currently not resolved how to interpret NSW for left
-        // shift by BitWidth - 1, so we avoid applying flags in that
-        // case. Remove this check (or this comment) once the situation
-        // is resolved. See
-        // http://lists.llvm.org/pipermail/llvm-dev/2015-April/084195.html
-        // and http://reviews.llvm.org/D8890 .
```
But langref was fixed in rL286785, and the behavior is pretty obvious:
http://volta.cs.utah.edu:8080/z/MM4WZP
^ nuw can always be propagated. nsw can be propagated if
either nuw is specified, or the shift is by *less* than bitwidth-1.

This mimics similar D81189 Reassociate change, alive2 is happy about that one.

I'm not sure `NUW` isn't being printed, but that seems unrelated.

Reviewers: mkazantsev, reames, sanjoy, nlopes, craig.topper, efriedma

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81243