[NFC] SwitchInst: Introduce wrapper for prof branch_weights handling

[NFC] SwitchInst: Introduce wrapper for prof branch_weights handling

This patch introduces a wrapper class that re-implements
several mutator methods of SwitchInst to handle changes
of prof branch_weights metadata along with remove/add
switch case methods.
Subsequent patches will use this wrapper to implement
prof branch_weights metadata handling for SwitchInst.

Reviewers: davidx, eraman, reames, chandlerc
Reviewed By: davidx
Differential Revision: https://reviews.llvm.org/D62122

llvm-svn: 361596