[llvm] [RISCV][TTI] Enable masked interleave access for scalable vector (PR #149981)

[Alex Bradbury via llvm-commits]

asb wrote:

This patch has caused an assertion on the zvl1024b builder [[1]](https://lab.llvm.org/buildbot/#/builders/213/builds/112) (actually the first zvl-specific failure the recently added builders have caught I think, so good to see the infra paying off):
```
clang-21: ../../llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7132: VectorizationFactor llvm::LoopVectorizationPlanner::computeBestVF(): Assertion `(BestFactor.Width == LegacyVF.Width || BestPlan.hasEarlyExit() || planContainsAdditionalSimplifications(getPlanFor(BestFactor.Width), CostCtx, OrigLoop, BestFactor.Width) || planContainsAdditionalSimplifications( getPlanFor(LegacyVF.Width), CostCtx, OrigLoop, LegacyVF.Width)) && " VPlan cost model and legacy cost model disagreed"' failed.
```

I'm reverting it now so the issue can be resolved without undue time pressure, but this minimised test case should help:
```
cat - <<'EOF' > reduced.ll
; ModuleID = '<stdin>'
source_filename = "<stdin>"
target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
target triple = "riscv64-unknown-linux-gnu"

define ptr @ham(ptr %arg) {
bb:
  store ptr null, ptr %arg, align 8
  ret ptr null
}

define ptr @zot(i64 %arg) {
bb:
  br label %bb1

bb1:                                              ; preds = %bb4, %bb
  %phi = phi ptr [ null, %bb ], [ %getelementptr, %bb4 ]
  %phi2 = phi i64 [ %arg, %bb ], [ %add, %bb4 ]
  %icmp = icmp sgt i64 %phi2, 0
  br i1 %icmp, label %bb4, label %bb3

bb3:                                              ; preds = %bb1
  ret ptr null

bb4:                                              ; preds = %bb1
  %call = call ptr @ham(ptr %phi)
  %getelementptr = getelementptr nusw i8, ptr %phi, i64 16
  %add = add i64 %phi2, -1
  br label %bb1
}
EOF
./build/rvrel/bin/clang++ --target=riscv64-linux-gnu -march=rva23u64_zvl1024b -O3 reduced.ll 
```

https://github.com/llvm/llvm-project/pull/149981