Add reduce_add() for int8 and int16 types.

This maps to specialized instructions (e.g. PSADBW) when available.

builtins/target-generic-1.ll

@@ -471,6 +471,15 @@ define  i64 @__popcnt_int64(i64) nounwind readonly alwaysinline {
   ret i64 %call
 }
 
+define i8 @__reduce_add_int8(<1 x i8> %v) nounwind readonly alwaysinline {
+  %r = extractelement <1 x i8> %v, i32 0
+  ret i8 %r
+}
+
+define i16 @__reduce_add_int16(<1 x i16> %v) nounwind readonly alwaysinline {
+  %r = extractelement <1 x i16> %v, i32 0
+  ret i16 %r
+}
 
 define  float @__reduce_add_float(<1 x float> %v) nounwind readonly alwaysinline {
   %r = extractelement <1 x float> %v, i32 0