live-in lists: new verification and isLiveOut() method

Ping! (this was sent originally on Oct 15*).*


I have proposed a patch to address the issue discussed previously about 
the live-in lists and their trustworthiness. In short, some 
optimizations depend on checking if a physical register is live out of 
MBB and this should preferably be the case only if that register is in 
one of MBBs successors live-in list. The SystemZ backend does this check 
in SystemZElimCompare.cpp:isCCLiveOut(), but it would have been much 
preferred to have this done by a method in common-code. Note that this 
is now intended for use for a non-allocatable, non-reserved register and 
does not depend on or help register allocation.

My patch adds a check for the live-through case of live-in lists in the 
MachineVerifier and moves the method that checks the live-in lists of 
the successors to MachineBasicBlock:

/// Return true if the specified physical register is in the live in set of
/// any of the successor blocks. If false is returned, Reg is guaranteed to
/// not be live out, although sub/super-registers are not checked. This can
/// be called both before and after regalloc.
bool isLiveOut(MCRegister PhysReg) const;

I would like to know if anyone would be in favor of having this here 
with a stated guarantee as in the comment, or if there are any 
objections to it?