[llvm] [Hexagon] Use word splats for repeated HVX build-vector words (PR #204808)

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L-roro wrote:

Thanks for taking the time to review this @iajbar .

> If you still see a miscompilation after this change, could you please share a reduced test case that reproduces the issue?

After this message I was double checking my code before sharing it and realized that I made a mistake on a c harness that I was using to validate the assembly. Therefore, I do not have any miscompilation to share. My apologies.

https://github.com/llvm/llvm-project/pull/204808