Use the new TRI->getLargestLegalSuperClass hook to constrain register class inflation.

Use the new TRI->getLargestLegalSuperClass hook to constrain register class inflation.

This has two effects: 1. We never inflate to a larger register class than what
the sub-target can handle. 2. Completely unconstrained virtual registers get the
largest possible register class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130229 91177308-0d34-0410-b5e6-96231b3b80d8