Make SwitchInstProfUpdateWrapper safer

While prof branch_weights inconsistencies are being fixed patch
by patch (pass by pass) we need SwitchInstProfUpdateWrapper to
be safe with respect to inconsistent metadata that can come from
passes that have not been fixed yet. See the bug found by @nikic
in https://reviews.llvm.org/D62126.

This patch introduces one more state (called Invalid) to the
wrapper class that allows users to work with the underlying
SwitchInst ignoring the prof metadata changes.

Created a unit test for the SwitchInstProfUpdateWrapper class.

Reviewers: davidx, nikic, eraman, reames, chandlerc
Reviewed By: davidx
Differential Revision: https://reviews.llvm.org/D62656

llvm-svn: 362473