Fix PR2748. Avoid coalescing physical register with virtual register which...

Fix PR2748. Avoid coalescing physical register with virtual register which would create illegal extract_subreg. e.g.
vr1024 = extract_subreg vr1025, 1
...
vr1024 = mov8rr AH
If vr1024 is coalesced with AH, the extract_subreg is now illegal since AH does not have a super-reg whose sub-register 1 is AH.

llvm-svn: 56118