[InstSimplify] delete shift-of-zero guard ops around funnel shifts

[InstSimplify] delete shift-of-zero guard ops around funnel shifts

This is a problem seen in common rotate idioms as noted in:
https://bugs.llvm.org/show_bug.cgi?id=34924

Note that we are not canonicalizing standard IR (shifts and logic) to the intrinsics yet.
(Although I've written this before...) I think this is the last step before we enable
that transform. Ie, we could regress code by doing that transform without this
simplification in place.

In PR34924, I questioned whether this is a valid transform for target-independent IR,
but I convinced myself this is ok. If we're speculating a funnel shift by turning cmp+br
into select, then SimplifyCFG has already determined that the transform is justified.
It's possible that SimplifyCFG is not taking into account profile or other metadata,
but if that's true, then it's a bug independent of funnel shifts.

Also, we do have CGP code to restore a guard like this around an intrinsic if it can't
be lowered cheaply. But that isn't necessary for funnel shift because the default
expansion in SelectionDAGBuilder includes this same cmp+select.

Differential Revision: https://reviews.llvm.org/D54552

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346960 91177308-0d34-0410-b5e6-96231b3b80d8