Backend: Fix GenRegister::offset sub reg offset
Backend: Fix GenRegister::offset sub reg offset

We used to ignore the reg.nr for subreg offset, but after GenRegister offset
is refined, we need to calculate the suboffset with nr and subnr.

Signed-off-by: Pan Xiuli <e5a665b1e626c5dc0826076a8b5a209eb48bf6ee@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet