We were leaking memory for uniforms that were previously defined but
don't have a location, e.g. structures.

This change also verifies that such uniforms have the same type in both
shaders. Also, simplify uniform lookup.

Bug chromium:863682

Change-Id: I468aace4df6f5329dc7bb9f33bf9bf533a743ae1
Reviewed-on: https://swiftshader-review.googlesource.com/19928Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>