[RDF] Simplify construction of maximal registers

[RDF] Simplify construction of maximal registers

The old algoritm was not correct, although it worked most of the time.
Avoid the complex reachability analysis and simply calculate the maximal
registers out of the set of all referenced registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314991 91177308-0d34-0410-b5e6-96231b3b80d8