[dev.simd] simd: add emulations for bitwise ops and for mask/merge methods

This CL adds the emulations under a "wrong name"; subsequent CLs
will move the AVX512 versions of these operations out of the way,
and then will rename these to their better names.

Change-Id: I49e7a73e4fea74fb7bd26cb8062014568d7999ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/692217
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/simd/simd_test.go

@@ -382,3 +382,17 @@ func TestBitMaskToBits(t *testing.T) {
 		t.Errorf("Want 0b101, got %b", v)
 	}
 }
+
+func TestMergeFloat(t *testing.T) {
+	a := simd.LoadFloat64x4Slice([]float64{1, 2, 3, 4})
+	b := simd.LoadFloat64x4Slice([]float64{4, 2, 3, 1})
+	g := a.Greater(b)
+	k := make([]int64, 4, 4)
+	g.AsInt64x4().StoreSlice(k)
+	checkSlices[int64](t, k, []int64{0, 0, 0, -1})
+	c := a.Merge(b, g)
+
+	s := make([]float64, 4, 4)
+	c.StoreSlice(s)
+	checkSlices[float64](t, s, []float64{4, 2, 3, 4})
+}