`LLVM_HAS_NVPTX_TARGET` is already defined in `llvm/Config/Targets.h`
and used to gate NVPTX-related code in MLIR. The same macro exists for
AMDGPU as `LLVM_HAS_AMDGPU_TARGET`, but MLIR defined its own
`MLIR_ENABLE_ROCM_CONVERSIONS` variable for this purpose. This PR
removes `MLIR_ENABLE_ROCM_CONVERSIONS` and replaces it with
`LLVM_HAS_AMDGPU_TARGET`, bringing parity with the NVPTX target.
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Co-authored-by: William Moses <gh@wsmoses.com>
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