R600/SI: Add exec_lo and exec_hi subregisters.

This allows accessing an SReg subregister with a normal subregister
index, instead of getting a machine verifier error.

Also be sure to include all of these subregisters in SReg_32.
This fixes inferring SGPR instead of SReg when finding a
super register class.
R600/SI: Use register class instead of list of registers

I'm not sure if this has any consequence or not.