[llvm] [SPIRV] Requires SCFG for Vulkan's SPIR-V (PR #67441)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

TBH, I am not convinced this is the right solution.
Seems like having a condition here controlling this generic behavior while this is only required for SPIR-V seems wrong.
But checking for a specific target in this code also feels weird.

Leaving this as draft for now, until we figured out a path forward for a structurizer.

https://github.com/llvm/llvm-project/pull/67441