add the Vector16 capability for both vec8 and vec16 types (KhronosGroup#229)

* add the Vector16 capability for both vec8 and vec16 types

* added test to verify Vector16 capability is added for vectors with eight elements

Author: bashbaug

libSPIRV/SPIRVType.h

@@ -281,7 +281,9 @@ class SPIRVTypeVector:public SPIRVType {
   bool isValidIndex(SPIRVWord Index) const { return Index < CompCount;}
   SPIRVCapVec getRequiredCapability() const {
     SPIRVCapVec V(getComponentType()->getRequiredCapability());
-    if (CompCount > 8)
+    // Even though the capability name is "Vector16", it describes
+    // usage of 8-component or 16-component vectors.
+    if (CompCount >= 8)
       V.push_back(CapabilityVector16);
     return std::move(V);
   }