Add a TRI::getLargestLegalSuperClass hook to provide an upper limit on register class...

Add a TRI::getLargestLegalSuperClass hook to provide an upper limit on register class inflation.

The hook will be used by the register allocator when recomputing register
classes after removing constraints.

Thumb1 code doesn't allow anything larger than tGPR, and x86 needs to ensure
that the spill size doesn't change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130228 91177308-0d34-0410-b5e6-96231b3b80d8