IR: Add a broad bitcode compatibility test

Successive versions of LLVM should retain the ability to parse bitcode
generated by old releases of the compiler.  This adds a bitcode format
compatibility test, which is intended to provide good (albeit not
entirely exhaustive) coverage of the current LangRef.

This also includes compatibility tests for LLVM 3.6.  After every 3.X.0
release, the compatibility.ll file from the 3.X branch should be copied
to compatibility-3.X.ll on trunk, and the 3.X.0 release used to generate
a corresponding bitcode file.

Patch by Vedant Kumar!

llvm-svn: 243779