[GPUToSPIRV][BF16] Handle single element vector case for bf16 emulation.

[mshahneo]

bf16 emulation requires that we match certain op patterns created by the bf16-to-gpu pass and insert Intel BF16 SPIR-V convert op in the GPUToSPIRV pass.

However, previously, single-element vector was not supported. This patch adds the support.

Remove imex-remove-single-elem-vector from the pipeline.

Add upstream single element vector{load|store} handling patch. This patch should be removed in next LLVM version update.

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