R600/SI: Use correct dest register class for V_READFIRSTLANE_B32

This instructions writes to an 32-bit SGPR.  This change required adding
the 32-bit VCC_LO and VCC_HI registers, because the full VCC register
is 64 bits.

This fixes verifier errors on several of the indirect addressing piglit
tests.

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 204055