Fix PR850 and CodeGen/X86/2006-07-31-SingleRegClass.ll.

The CFE refers to all single-register constraints (like "A") by their 16-bit
name, even though the 8 or 32-bit version of the register may be needed.
The X86 backend should realize what is going on and redecode the name back
to its proper form.

llvm-svn: 29420