Add support for predicated vector adds.  Basically, the pattern:

	%newval = add <16 x i32> %x, %y
	%z = select <16 x i1> %mask, <16 x i32> %newval, <16 x i32> %oldval

Can generate a predicated op:

   v0{s1} = v2 + v3

By using a constraint that indicates %oldval and %newval are equal.