[XLA:CPU] Re-use the same llvm::GlobalVariable for identical literals

[XLA:CPU] Re-use the same llvm::GlobalVariable for identical literals

This isn't necessary today, but it will be after an optimization change I'm
about to make.

LLVM has a constant merging pass too, but one of the motivations here is to
avoid the LLVM compile time overhead of having many large arrays in the IR.

PiperOrigin-RevId: 195032900