llvmpipe: fix quad group helper invocation masking
LLVMpipe doesn't implement quad group helper invocations properly.
More specifically:
- Handle output variables correctly
- Don't apply the rasterizer mask to global and SSBO memory reads
There is a separate MR to add tests: !31465.
Here are a couple of USD test results to illustrate the problem.
Edited by Aleksi Sapon