[llvm] [LLVM][LangRef] Redefine out-of-range stepvector values as being truncated. (PR #173494)

[Paul Walker via llvm-commits]

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paulwalker-arm wrote:

> Why not drop the if entirely here instead of keeping the nullptr return?

That's point two in the message.  This whole functionality belongs in simplifyExtractElementInst, so I figured it's better to do the work once in a separate PR?

https://github.com/llvm/llvm-project/pull/173494