[llvm] [RISCV][GlobalISel] Vector Extension vadd Legalizer (PR #71400)

[Michael Maitland via llvm-commits]

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+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
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michaelmaitland wrote:

Also because `vadd` is not the GISel opcode we are legalize. We are legalizing a G_ADD opcode that operates on vector types.

https://github.com/llvm/llvm-project/pull/71400