Rename GR8_ABCD to GR8_ABCD_L and create GR8_ABCD_H, and use these

to precisely describe the h-register subreg register classes.
Thanks to Jakob Stoklund Olesen for spotting this and for the
initial patch!

Also, make getStoreRegOpcode and getLoadRegOpcode aware of the
needs of h registers.

llvm-svn: 70211