Fix a translator crash with index expressions.

This crash happened with certain bad shaders which used temporary
values as array (or other) index expresisons.

Fixes the crash covered in the WebGL test
"conformance/bugs/undefined-index-should-not-crash"

BUG=angle:857

Change-Id: I13e2ba6d5f1ab0846ac902021bc0b57cbb37d759
Reviewed-on: https://chromium-review.googlesource.com/237460
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>