Teach 2addr pass to be do more commuting. If both uses of a two-address...

Teach 2addr pass to be do more commuting. If both uses of a two-address instruction are killed, but the first operand has a use before and after the def, commute if the second operand does not suffer from the same issue.
%reg1028<def> = EXTRACT_SUBREG %reg1027<kill>, 1                                                                                                                                     
%reg1029<def> = MOV8rr %reg1028                                                                                                                                                      
%reg1029<def> = SHR8ri %reg1029, 7, %EFLAGS<imp-def,dead>                                                                                                                            
insert => %reg1030<def> = MOV8rr %reg1028                                                                                                                                            
%reg1030<def> = ADD8rr %reg1028<kill>, %reg1029<kill>, %EFLAGS<imp-def,dead>                                                                                                         

In this case, it might not be possible to coalesce the second MOV8rr                                                                                                                 
instruction if the first one is coalesced. So it would be profitable to                                                                                                              
commute it:                                                                                                                                                                          
%reg1028<def> = EXTRACT_SUBREG %reg1027<kill>, 1                                                                                                                                     
%reg1029<def> = MOV8rr %reg1028                                                                                                                                                      
%reg1029<def> = SHR8ri %reg1029, 7, %EFLAGS<imp-def,dead>                                                                                                                            
insert => %reg1030<def> = MOV8rr %reg1029                                                                                                                                            
%reg1030<def> = ADD8rr %reg1029<kill>, %reg1028<kill>, %EFLAGS<imp-def,dead>

llvm-svn: 62954