Fixed uniform block indexing

A uniform block's qualifier is generally a uniform, so
we have to change that to a temporary in order to be
able to us it as the result of the indexing operation.

Change-Id: I2f5ba7f966e8eaab314322edfa0483a9f24bfbdb
Reviewed-on: https://swiftshader-review.googlesource.com/4543
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>