Fix a bug in spill weight computation. If the alias is a super-register, and...

Fix a bug in spill weight computation. If the alias is a super-register, and the super-register is in the register class we are trying to allocate. Then add the weight to all sub-registers of the super-register even if they are not aliases.                                                                                                  
e.g. allocating for GR32, bh is not used, updating bl spill weight.                                                                                                        
     bl should get the same spill weight otherwise it will be choosen                                                                                              
     as a spill candidate since spilling bh doesn't make ebx available.
This fix PR2866.

llvm-svn: 67574