Avoid left shift of negative value in soft-fp16

Source/UnitTest/test_softfloat.cpp

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+// SPDX-License-Identifier: Apache-2.0
+// ----------------------------------------------------------------------------
+// Copyright 2020-2021 Arm Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy
+// of the License at:
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations
+// under the License.
+// ----------------------------------------------------------------------------
+
+/**
+ * @brief Unit tests for the vectorized SIMD functionality.
+ *
+ * This test suite is a partial implementation, focussing on 4-wide vectors.
+ * We're adding things as we touch related parts of the code, but there is some
+ * technical debt to catch up on to get full coverage.
+ */
+
+#include <limits>
+
+#include "gtest/gtest.h"
+
+#include "../astcenc_internal.h"
+#include "../astcenc_vecmathlib.h"
+
+namespace astcenc
+{
+
+#if (ASTCENC_F16C == 0) && (ASTCENC_NEON == 0)
+
+/** @brief Test normal numbers. */
+TEST(softfloat, FP16NormalNumbers)
+{
+	float result = sf16_to_float((15 << 10) + 1);
+	EXPECT_NEAR(result,  1.00098f, 0.00005f);
+}
+
+/** @brief Test VLA loop limit round down. */
+TEST(softfloat, FP16DenormalNumbers)
+{
+	float result = sf16_to_float((0 << 10) + 1);
+	EXPECT_NEAR(result, 5.96046e-08f, 0.00005f);
+}
+
+/** @brief Test zero. */
+TEST(softfloat, FP16Zero)
+{
+	float result = sf16_to_float(0x0000);
+	EXPECT_EQ(result, 0.0f);
+}
+
+/** @brief Test NaN. */
+TEST(softfloat, FP16NaN)
+{
+	float result = sf16_to_float(0xFFFF);
+	EXPECT_FALSE(result == result);
+}
+
+#endif
+
+}