The ESSL output doesn't have a default precision for floats, this causes
float literal statements to not have any precision defined, which is an
error. We fix this by removing literal statements as they are dead code
anyway.

BUG=angleproject:1967

Change-Id: I498f4f8495f854240ee8a2182415bf982c5166a4
Reviewed-on: https://chromium-review.googlesource.com/470268Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>