gallivm: fix somewhat broken NaN behavior for exp2

gallivm: fix somewhat broken NaN behavior for exp2

I actually screwed that up in 754319490f6946a9ad5ee619822d5fe4254e6759,
mistakenly thinking the code actually wanted the non-nan result before.
So, introduce that missing nan behavior case and use that instead.
For sse, there's no actual change in the resulting code at all, the fallback
code wouldn't have done the right thing though.
Of course, the actual issue I saw with pow() was completely unrelated...

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>