HLSL: allow destination swizzles when writing RWTexture/RWBuffer objects.

Reads and write syntax to UAV objects is turned into EOpImageLoad/Store
operations.  This translation did not support destination swizzles,
for example, "mybuffer[tc].zyx = 3;", so such statements would fail to
compile.  Now they work.

Parial updates are explicitly prohibited.

New test: hlsl.rw.swizzle.frag

Test/hlsl.rw.swizzle.frag

@@ -0,0 +1,28 @@
+RWTexture2D<float3> rwtx;
+RWBuffer<float3> buf;
+
+float3 SomeValue() { return float3(1,2,3); }
+
+float4 main() : SV_Target0
+{
+    int2 tc2 = { 0, 0 };
+    int tc = 0;
+
+    // Test swizzles and partial updates of L-values when writing to buffers and writable textures.
+    rwtx[tc2].zyx = float3(1,2,3);     // full swizzle, simple RHS
+    rwtx[tc2].zyx = SomeValue();       // full swizzle, complex RHS
+    rwtx[tc2].zyx = 2;                 // full swizzle, modify op
+
+    // Partial updates not yet supported.
+    // Partial values, which will use swizzles.
+    // buf[tc].yz = 42;                 // partial swizzle, simple RHS
+    // buf[tc].yz = SomeValue().x;      // partial swizzle, complex RHS
+    // buf[tc].yz += 43;                // partial swizzle, modify op
+
+    // // Partial values, which will use index.
+    // buf[tc].y = 44;                  // single index, simple RHS
+    // buf[tc].y = SomeValue().x;       // single index, complex RHS
+    // buf[tc].y += 45;                 // single index, modify op
+
+    return 0.0;
+}