There was a typo in the vertex shader cache which could trigger an
overflow bug.

TEST=dEQP-GLES3.functional.ubo.random.scalar_types.*
BUG=angleproject:1077

Change-Id: I5652cf9675155b627f84531e09c01b42e29278fc
Reviewed-on: https://chromium-review.googlesource.com/286775Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>