DXC is producing duplicate definitions for local variables.
Give more context in the `ASSERT()` so it's easier to diagnose whats going wrong.

Bug: b/170378370
Change-Id: Ic8c8f24a6dbd8ce23e04eb4f0265363b44bc174c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49128
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>