Backport #8259 to release/17.x

[steven-johnson]

• Fix Vulkan SIMT mappings for GPU loop vars. Previous refactoring accidentally used the fully qualified var name rather than the categorized vulkan intrinsic name.

• Avoid formatting the GPU kernel to a string for Vulkan (since it's binary SPIR-V needs to remain intact).

[steven-johnson]

Looks like there are Vulkan failures -- any clues? Are there more fixes I missed?

[derek-gerstmann]

Sorry! Yes, I saw this as well, but haven't been able to reproduce this on my Linux machine. Still investigating.

[derek-gerstmann]

We should also grab: https://github.com/halide/Halide/pull/8130

[steven-johnson]

PTAL -- lots of conflicts in the cherry pick so hopefully I did it right

[derek-gerstmann]

It looks like a few things from main got left behind in src/runtime/internal. I'll add those to this PR.

[derek-gerstmann]

I inspected buildbot-4 and it only had v1.2 of the Vulkan SDK installed, but the build target we're using is generating code for SDK v1.3. I've upgraded that machine, but the other linux workers will likely need to get upgraded too.

[steven-johnson]

the other linux workers will likely need to get upgraded too.

Tell me the process to upgrade and I'll handle it

[derek-gerstmann]

The packaging has changed since we originally setup the machines, and now everything is bundled under the vulkan-sdk for Ubuntu:

https://vulkan.lunarg.com/doc/view/latest/linux/getting_started_ubuntu.html

I just needed:

• Ubuntu 20.04 (Focal Fossa)

wget -qO - http://packages.lunarg.com/lunarg-signing-key-pub.asc | sudo apt-key add -
sudo wget -qO /etc/apt/sources.list.d/lunarg-vulkan-focal.list http://packages.lunarg.com/vulkan/lunarg-vulkan-focal.list
sudo apt update
sudo apt install vulkan-sdk

That should make vulkaninfo report SDK v1.3.x