[Metal] Metal unsupported 64 bit wave op when using WavePrefixSum

[kcloudy0717]

WavePrefixSum.int64.test in #934 is failing on metal. See https://github.com/llvm/offload-test-suite/actions/runs/22891383436/job/66501367356?pr=934#step:12:402 for more info.

stdout:

metal-shaderconverter: [callSIMDOp:6647] Fatal: Unsupported 64bit wave op