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.

llvm-svn: 124365