[llvm] [SPIR-V] Add pre-headers to loops. (PR #75844)

[Ilia Diachkov via llvm-commits]

iliya-diyachkov wrote:

> @iliya-diyachkov don't know if you want to review, but FYI as this could changes the current codegen

@Keenuts , do you think we should structure CFG for all SPIR-V flavors (both graphics and OpenCL)? If not, we probably need to call removeImplicitFallthroughs() and run createLoopSimplifyPass, checking for some flag or extension (or maybe isOpenCLEnv() from SPIRVSubtarget)?

https://github.com/llvm/llvm-project/pull/75844