[RISCV] Vectorize phi for loop carried @llvm.vp.reduce.* (#131974)

LLVM vector predication reduction intrinsics return a scalar result, but
on RISC-V vector reduction instructions write the result in the first
element of a vector register. So when a reduction in a loop uses a
scalar phi, we end up with unnecessary scalar moves:
```asm
loop:
    vmv.s.x v8, zero
    vredsum.vs v8, v10, v8
    vmv.x.s a0, v8
````
This mainly affects vector predication reduction. This tries to
vectorize any scalar phis that feed into a vector predication reduction
in RISCVCodeGenPrepare, converting:
```llvm
vector.body:
%red.phi = phi i32 [ ..., %entry ], [ %red, %vector.body ]
%red = tail call i32 @llvm.vp.reduce.add.nxv4i32(i32 %red.phi, <vscale x 4 x i32> %wide.load, <vscale x 4 x i1> splat (i1 true), i32 %evl)
```
to
```llvm
vector.body:
%red.phi = phi <vscale x 2 x i32> [ ..., %entry ], [ %acc.vec, %vector.body]
%phi.scalar = extractelement <vscale x 2 x i32> %red.phi, i64 0
%acc = tail call i32 @llvm.vp.reduce.add.nxv4i32(i32 %phi.scalar, <vscale x 4 x i32> %wide.load, <vscale x 4 x i1> splat (i1 true), i32 %evl)
%acc.vec = insertelement <vscale x 2 x i32> poison, float %acc, i64 0
```
Which eliminates the scalar -> vector -> scalar crossing during
instruction selection.

---------

Co-authored-by: yanming <ming.yan@terapines.com>
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tree: cd0880fd36544c160c70d041e258bbd8b439cc76
  1. .ci/
  2. .github/
  3. bolt/
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  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
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  11. libc/
  12. libclc/
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  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
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  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
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