Changed the response to a bit 'zero' in the exec mask

[d1duarte]

Allows the exec mask to have one bit set to zero without immediately stopping the memory instruction. This way the zero bits are skipped but the instruction keeps performing until the end of the mask is reached.

[zwabbit]

I'm not actually sure that condition is possible due to the way the ISA and the microarchitecture is set up. The way I understand the execute mask, the first time you run into a zero bit, all of the bits higher then it are zero, as the compute unit only enables vector ALUs consecutively and not alternating.

[d1duarte]

I'm not sure how this would work on a thread divergence scenario. I suggest we leave this change out for now. If, in the future, we find it necessary we'll simply add it then.

[zwabbit]

I actually had not thought of the thread divergence issue. And I also don't have any idea what happens in that case. So yeah, let's punt until we can dig up some actual info.