[LV][VPlan] Reduce register usage of VPEVLBasedIVPHIRecipe. (#154482)

`VPEVLBasedIVPHIRecipe` will lower to VPInstruction scalar phi and
generate scalar phi. This recipe will only occupy a scalar register just
like other phi recipes.

This patch fix the register usage for `VPEVLBasedIVPHIRecipe` from
vector
to scalar which is close to generated vector IR.

https://godbolt.org/z/6Mzd6W6ha shows that no register spills when
choosing `<vscale x 16>`.

Note that this test is basically copied from AArch64.

Author: ElvisWang123

llvm/lib/Transforms/Vectorize/VPlanAnalysis.cpp

@@ -555,7 +555,7 @@ SmallVector<VPRegisterUsage, 8> llvm::calculateRegisterUsageForPlan(
 
         if (VFs[J].isScalar() ||
             isa<VPCanonicalIVPHIRecipe, VPReplicateRecipe, VPDerivedIVRecipe,
-                VPScalarIVStepsRecipe>(R) ||
+                VPEVLBasedIVPHIRecipe, VPScalarIVStepsRecipe>(R) ||
             (isa<VPInstruction>(R) &&
              vputils::onlyScalarValuesUsed(cast<VPSingleDefRecipe>(R))) ||
             (isa<VPReductionPHIRecipe>(R) &&

llvm/test/Transforms/LoopVectorize/RISCV/reg-usage-maxbandwidth.ll

@@ -0,0 +1,33 @@
+; REQUIRES: asserts
+; RUN: opt -passes=loop-vectorize -mtriple riscv64 -mattr=+v -vectorizer-maximize-bandwidth -debug-only=loop-vectorize,vplan -disable-output -S < %s 2>&1 | FileCheck %s --check-prefixes=CHECK-REGS-VP
+
+define i32 @dotp(ptr %a, ptr %b) {
+; CHECK-REGS-VP:      LV(REG): VF = vscale x 16
+; CHECK-REGS-VP-NEXT: LV(REG): Found max usage: 2 item
+; CHECK-REGS-VP-NEXT: LV(REG): RegisterClass: RISCV::GPRRC, 6 registers
+; CHECK-REGS-VP-NEXT: LV(REG): RegisterClass: RISCV::VRRC, 24 registers
+; CHECK-REGS-VP-NEXT: LV(REG): Found invariant usage: 1 item
+; CHECK-REGS-VP-NEXT: LV(REG): RegisterClass: RISCV::GPRRC, 1 registers
+; CHECK-REGS-VP:      LV: Selecting VF: vscale x 16.
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %for.body ]
+  %accum = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  %gep.a = getelementptr i8, ptr %a, i64 %iv
+  %load.a = load i8, ptr %gep.a, align 1
+  %ext.a = zext i8 %load.a to i32
+  %gep.b = getelementptr i8, ptr %b, i64 %iv
+  %load.b = load i8, ptr %gep.b, align 1
+  %ext.b = zext i8 %load.b to i32
+  %mul = mul i32 %ext.b, %ext.a
+  %sub = sub i32 0, %mul
+  %add = add i32 %accum, %sub
+  %iv.next = add i64 %iv, 1
+  %exitcond.not = icmp eq i64 %iv.next, 1024
+  br i1 %exitcond.not, label %for.exit, label %for.body
+
+for.exit:                        ; preds = %for.body
+  ret i32 %add
+}