Add UAV (image) binding offset and HLSL register support

This PR adds:

1. The "u" register class for RW* objects.

2. --shift-image-bindings (== --sib), analogous to --shift-texture-bindings etc.

3. Case insensitive reg classes.

4. Tests for above.

Test/spv.rw.autoassign.frag

@@ -0,0 +1,18 @@
+
+RWTexture1D <float> g_tTex1df1;
+RWBuffer <uint>     g_tBuf1du1;
+
+struct PS_OUTPUT
+{
+    float4 Color : SV_Target0;
+};
+
+PS_OUTPUT main()
+{
+    float r00 = g_tTex1df1[0];
+    uint  r01 = g_tBuf1du1[0];
+
+    PS_OUTPUT psout;
+    psout.Color = 0;
+    return psout;
+}