[dev.simd] cmd/compile, simd: support load from bits for mask

This CL is partially generated by CL 688855.

Change-Id: I68d5fbad9445a3d2cf671822be1c0b82e7290396
Reviewed-on: https://go-review.googlesource.com/c/go/+/688875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/simd/simd_test.go

@@ -460,3 +460,20 @@ func testMergeLocalswrapper(t *testing.T, op func(simd.Int64x4, simd.Int64x4) si
 		}
 	}
 }
+
+func TestBitMask(t *testing.T) {
+	if !simd.HasAVX512() {
+		t.Skip("Test requires HasAVX512, not available on this hardware")
+		return
+	}
+	var bits uint64 = 0b10
+	results := [2]int64{}
+	want := [2]int64{0, 6}
+	m := simd.LoadMask64x2FromBits(&bits)
+	simd.LoadInt64x2Slice([]int64{1, 2}).AddMasked(simd.LoadInt64x2Slice([]int64{3, 4}), m).Store(&results)
+	for i := range 2 {
+		if results[i] != want[i] {
+			t.Errorf("Result at %d incorrect: want %v, got %v", i, want[i], results[i])
+		}
+	}
+}