Get rid of a pseudo instruction and replace it with subreg based operation on...

Get rid of a pseudo instruction and replace it with subreg based operation on real instructions, ridding the asm printers of the hack used to do this previously. In the process, update LowerSubregs to be careful about eliminating copies that have side affects. 

Note: the coalescer will have to be careful about this too, when it starts coalescing insert_subreg nodes.
llvm-svn: 48329