HLSL output: Fix nested dynamic indexing in an l-value This fixes cases where a matrix is indexed and then the resulting vector is indexed in the same expression inside an l-value. BUG=angleproject:1672 TEST=angle_end2end_tests Change-Id: Ia0593d75da67e0aa582003e1c381f2a1c89f948c Reviewed-on: https://chromium-review.googlesource.com/427938 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>