Empty declarations are possible in loop init expressions. Make
PruneEmptyDeclarations take this into account.

BUG=angleproject:1550
TEST=angle_unittests

Change-Id: If407babf9b6f7a26dfcf73ff345493d3e2af3f9a
Reviewed-on: https://chromium-review.googlesource.com/401147Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>