Add a test for 8- and 16-bit construction.

Test/spv.8bit-16bit-construction.frag

@@ -0,0 +1,22 @@
+#version 450 core
+
+#extension GL_EXT_shader_8bit_storage : enable
+#extension GL_EXT_shader_16bit_storage : enable
+
+buffer B
+{
+    int8_t i8_from_i16;
+    int16_t i16_from_i8;
+    uint8_t u8_from_u16;
+    uint16_t u16_from_u8;
+    float16_t f16_from_i8;
+};
+
+void main()
+{
+    i8_from_i16 = int8_t(int16_t(1));
+    i16_from_i8 = int16_t(int8_t(1));
+    u8_from_u16 = uint8_t(uint16_t(1));
+    u16_from_u8 = uint16_t(uint8_t(1));
+    f16_from_i8 = float16_t(int8_t(1));
+}