[IR] SelectInst: add swapValues() utility

Summary:
Sometimes we need to swap true-val and false-val of a `SelectInst`.
Having a function for that is nicer than hand-writing it each time.

Reviewers: spatel, RKSimon, craig.topper, jdoerfert

Reviewed By: jdoerfert

Subscribers: jdoerfert, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65520

llvm-svn: 367547