[InstCombine] When turning sext into zext due to known bits, return the new ZExt...

[InstCombine] When turning sext into zext due to known bits, return the new ZExt instead of calling replaceinstuseswith

The worklist loop that we're returning back to should be able to do the repacement itself. This is how we normally do replacements.

My main motivation was that I observed that we weren't preserving the name of the result when we do this transform. The replacement code in the worklist loop will call takeName as part of the replacement.

Differential Revision: https://reviews.llvm.org/D61695

llvm-svn: 360284