Fix PR2343. An *interesting* coalescer bug.

BB1:                                                                                                                                                  
  vr1025 = copy vr1024                                                                                                                                
  ..                                                                                                                                                  
BB2:                                                                                                                                                  
  vr1024 = op                                                                                                                                         
         = op vr1025                                                                                                                                     
  <loop eventually branch back to BB1>

Even though vr1025 is copied from vr1024, it's not safe to coalesced them since live range of vr1025 intersects the def of vr1024. This happens when vr1025 is assigned the value of the previous iteration of vr1024 in the loop.

llvm-svn: 51394