[RFC] SPIR-V backend integration

Hi all,

Intel, Arm and Khronos are proposing to start the integration of SPIR-V backend to LLVM.

The general RFC was discussed previously in March 2021 (https://lists.llvm.org/pipermail/llvm-dev/2021-March/148905.html). This backend has been developed in the Khronos repository by Intel and Arm (https://github.com/KhronosGroup/LLVM-SPIRV-Backend). The code has been refactored so it does not require extra changes from GlobalISel or other LLVM target-independent infrastructure. We have added a pretty extensive LIT test suite for the backend functionality.

SPIR-V triple was upstreamed into the LLVM trunk in the middle of November (https://reviews.llvm.org/D109144). We have submitted for consideration the first patch (https://reviews.llvm.org/D115009) - this is a stub for the SPIR-V backend. Please feel free to review it. All comments are welcome.