## Description
This PR updates the `ConvertGpuOpsToLLVMSPVOps`'s option by replacing
the `index-bitwidth` with a boolean option `use-64bit-index` (similar to
the `ConvertGPUToSPIRV` option).
The reason for this modification is because the
`ConvertGpuOpsToLLVMSPVOps`:
> Generate LLVM operations to be ingested by a SPIR-V backend for gpu
operations
In the context of SPIR-V specifications only two physical addressing
models are allowed: `Physical32` and `Physical64`.
This change guarantees output sanity by preventing invalid or
unsupported index bitwidths from being specified.
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