[llvm] Support non-constant `G_BUILD_VECTOR` in X86. (PR #182502)

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YohayAiTe wrote:

Due to IRL constraints I could not work on this PR as much as I would like to, so I wanted to update in case anyone wants to pick it up, and to see if anyone has any thoughts to share.

My current approach is to move most of the logic to the instruction selection, and copy most of the logic from the ISD, where it is done using c++ (rather than tablegen) since the logic is quite complex. The problem arises when the selection can have multiple valid options with different domains (e.g. a function that builds a vector from GPRs can be selected as `movd`+`insertps` or `

https://github.com/llvm/llvm-project/pull/182502