Change weight array into a vector and make it as big as the number of

Change weight array into a vector and make it as big as the number of
registers (not as the max number of registers).

Change toSpill from a std::set into a std::vector<bool>.

Use the reverse iterator adapter to do a reverse scan of allocatable
registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11061 91177308-0d34-0410-b5e6-96231b3b80d8