Use the correct type for the constant for matrix/scalar division

When a matrix is divided by a scalar it tries to take the reciprocal
of the scalar to convert the operation into a multiply. However it was
always doing this by making a 32-bit constant. If the scalar is a
double then this would end up making an FDiv instruction with
different types in the operands.

This patch adds a helper method called makeFpConstant which makes a
floating-point constant of the given type. The code to take the
reciprocal now uses it to make the same type as the result.

Fixes https://github.com/KhronosGroup/glslang/issues/1278

SPIRV/SpvBuilder.h

@@ -226,6 +226,7 @@ public:
     Id makeFloatConstant(float f, bool specConstant = false);
     Id makeDoubleConstant(double d, bool specConstant = false);
     Id makeFloat16Constant(float f16, bool specConstant = false);
+    Id makeFpConstant(Id type, double d, bool specConstant = false);
 
     // Turn the array of constants into a proper spv constant of the requested type.
     Id makeCompositeConstant(Id type, const std::vector<Id>& comps, bool specConst = false);