In some generated shaders, "levels" would be read before it is assigned
by GetDimensions when clamping mip level to valid range. Fix these cases.

BUG=angleproject:1092
TEST=dEQP-GLES3.functional.shaders.texture_functions.*
     (12 tests start passing)

Change-Id: I9fce8d378606738e5172673d222bce7968e26789
Reviewed-on: https://chromium-review.googlesource.com/312022
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>