Track liveness of unallocatable, unreserved registers in machine DCE.

Reserved registers are unpredictable, and are treated as always live by machine
DCE.

Allocatable registers are never reserved, and can be used for virtual registers.

Unreserved, unallocatable registers can not be used for virtual registers, but
otherwise behave like a normal allocatable register. Most targets only have
the flag register in this set.

llvm-svn: 112649