[RISCV] Vectorize phi for loop carried @llvm.vp.reduce.* (llvm#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 <[email protected]>

Author: NexMing

llvm/lib/Target/RISCV/RISCVCodeGenPrepare.cpp

@@ -113,9 +113,10 @@ bool RISCVCodeGenPrepare::visitAnd(BinaryOperator &BO) {
 // vfredosum.vs v8, v8, v10
 // vfmv.f.s fa0, v8
 //
-// This mainly affects ordered fadd reductions, since other types of reduction
-// typically use element-wise vectorisation in the loop body. This tries to
-// vectorize any scalar phis that feed into a fadd reduction:
+// This mainly affects ordered fadd reductions and VP reductions that have a
+// scalar start value, since other types of reduction typically use element-wise
+// vectorisation in the loop body. This tries to vectorize any scalar phis that
+// feed into these reductions:
 //
 // loop:
 // %phi = phi <float> [ ..., %entry ], [ %acc, %loop ]
@@ -137,7 +138,8 @@ bool RISCVCodeGenPrepare::visitIntrinsicInst(IntrinsicInst &I) {
   if (expandVPStrideLoad(I))
     return true;
 
-  if (I.getIntrinsicID() != Intrinsic::vector_reduce_fadd)
+  if (I.getIntrinsicID() != Intrinsic::vector_reduce_fadd &&
+      !isa<VPReductionIntrinsic>(&I))
     return false;
 
   auto *PHI = dyn_cast<PHINode>(I.getOperand(0));