[llvm] [SPIR-V] Select int-to-int convert opcode from source signedness (PR #201116)

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

> should not we also change `llvm/test/CodeGen/SPIRV/transcoding/OpenCL/convert_functions.ll`?

Source sign and destination sign matched there, so the previous fix is still applicable there, and the tests are passing

https://github.com/llvm/llvm-project/pull/201116