HLSL: Add EOpMatrixSwizzle, selectively decomposed to other ops, for issue #670.

Since EOpMatrixSwizzle is a new op, existing back-ends only work when the
front end first decomposes it to other operations. So far, this is only
being done for simple assignment into matrix swizzles.

Test/hlsl.matrixSwizzle.vert

@@ -2,9 +2,32 @@ void ShaderFunction(float inf) : COLOR0
 {
     float3x4 m;
 
+    // tests that convert to non-matrix swizzles
+
     m._34  = 1.0; // AST should have a normal component select
     m._m23 = 2.0; // same code
+    m[2][3] = 2.0; // same code
 
     m._11_12_13_14 = float4(3.0);      // AST should have normal column selection (first row)
     m._m10_m11_m12_m13 = float4(3.0);  // AST should have normal column selection (second row)
+    m[1] = float4(3.0);                // same code
+
+    // tests that stay as matrix swizzles
+
+    float3 f3;
+    m._11_22_23 = f3;
+    m._21_12_31 = float3(5.0);
+    m._11_12_21 = 2 * f3;
+
+    // r-value
+    f3 = m._21_12_31;
+}
+
+float3x3 createMat3x3(float3 a, float3 b, float3 c)
+{
+    float3x3 m;
+    m._11_21_31 = a;
+    m._12_22_32 = b;
+    m._13_23_33 = c;
+    return m;
 }