[llvm] [DirectX][LongVec] Support large vector splitting for dot product (PR #216383)

[Kaitlin Peng via llvm-commits]

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+; RUN: opt -S -dxil-intrinsic-expansion -mtriple=dxil-pc-shadermodel6.3-library %s | FileCheck %s --check-prefixes=CHECK,EXPCHECK
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kmpeng wrote:

Would it be worthwhile to add a multiple of 4 case (e.g. `<8 x float>` -> `dot4` + `dot4`) to exercise the no tail `std::min(Remaining, 4u)` path?

https://github.com/llvm/llvm-project/pull/216383