gallivm: eliminate a unnecessary AND with unorm lerps

gallivm: eliminate a unnecessary AND with unorm lerps

Instead of doing a add and then mask out the upper bits, we can
simply do a add with a half wide type (this, of course, assumes
the hw can actually do it...), so we'll get the required zero
in the upper bits automatically.

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