For DebugValue with index parameter, the debugger code does not use the
correct member index. It results in "undefined" value even when the
shader actually provide the value.

Bug: b/148401179
Change-Id: Ic563db74503200c1a6dc71a5e1744b75df63d610
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50072
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Jaebaek Seo <jaebaek@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>