[llvm] [VectorCombine][TTI] Disable Load Widening for SPIR-V (PR #227623)

[Gregory Roth via llvm-commits]

pow2clk wrote:

> > > Can't you just adjust the costs with SPIRVTTIImpl::getMemoryOpCost?

I'm afraid this may not actually have sufficient information to determine that the changed load is invalid. It's not that vectors can't be loaded, they just can't be loaded from a scalar pointer type in SPIR-V. The calls to calculate the [NewCost](https://github.com/llvm/llvm-project/blob/d9b12f5c63ff211229a1809a6878c32fc166c14a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp#L334)  only includes the opcode and type. If I passed in the original instruction as the optional parameter, I could compare those, but I don't think that's how that parameter is meant to work.

Note that the newest commit makes no changes based on this discussion, but just fixes the undefs in the test.

https://github.com/llvm/llvm-project/pull/227623