R600: Make sign_extend_inreg legal.

Don't know why I didn't just do this in the first place.
[PECOFF] Better handling of /OPT option.

Previously LLD would fail if /OPT:icf, /OPT:lbr or such are specified,
because these command line flags would be handled as unknown ones. We
rather want LLD to ignore these known but yet-to-be-implemented options
for now.

Added tests for the driver as well.
R600: Change how vector truncating stores are packed.

Don't introduce new operations on an illegal sub 32-bit type.
Do the operations on a 32-bit value, and then use a truncating store.