[RISCV] Enable prefetching writes (#130561)

We should prefetch writes since `Zicbop` has `prefetch.w`.

Author: wangpc-pp

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -354,6 +354,7 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
   unsigned getMaxPrefetchIterationsAhead() const override {
     return TuneInfo->MaxPrefetchIterationsAhead;
   };
+  bool enableWritePrefetching() const override { return true; }
 
   unsigned getMinimumJumpTableEntries() const;
 

llvm/test/Transforms/LoopDataPrefetch/RISCV/basic.ll

@@ -11,12 +11,16 @@ define void @foo(ptr nocapture %a, ptr nocapture readonly %b) {
 ; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = shl nuw nsw i64 [[INDVARS_IV]], 3
 ; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[TMP0]], 64
-; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, ptr [[B]], i64 [[TMP1]]
+; CHECK-NEXT:    [[SCEVGEP1:%.*]] = getelementptr i8, ptr [[A]], i64 [[TMP1]]
+; CHECK-NEXT:    [[TMP4:%.*]] = shl nuw nsw i64 [[INDVARS_IV]], 3
+; CHECK-NEXT:    [[TMP3:%.*]] = add i64 [[TMP4]], 64
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, ptr [[B]], i64 [[TMP3]]
 ; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds double, ptr [[B]], i64 [[INDVARS_IV]]
 ; CHECK-NEXT:    call void @llvm.prefetch.p0(ptr [[SCEVGEP]], i32 0, i32 3, i32 1)
 ; CHECK-NEXT:    [[TMP2:%.*]] = load double, ptr [[ARRAYIDX]], align 8
 ; CHECK-NEXT:    [[ADD:%.*]] = fadd double [[TMP2]], 1.000000e+00
 ; CHECK-NEXT:    [[ARRAYIDX2:%.*]] = getelementptr inbounds double, ptr [[A]], i64 [[INDVARS_IV]]
+; CHECK-NEXT:    call void @llvm.prefetch.p0(ptr [[SCEVGEP1]], i32 1, i32 3, i32 1)
 ; CHECK-NEXT:    store double [[ADD]], ptr [[ARRAYIDX2]], align 8
 ; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[INDVARS_IV_NEXT]], 1600