Avoid modifying the OneClassForEachPhysReg map while iterating over it.

Avoid modifying the OneClassForEachPhysReg map while iterating over it.
Linear scan regalloc is currently assuming that any register aliased with
a member of a regclass must also be in at least one regclass.  That is not
always true.  For example, for X86, RIP is in a regclass but IP is not.
If you're unlucky, this can cause a crash by invalidating the iterator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124365 91177308-0d34-0410-b5e6-96231b3b80d8