[RISCV] Preserve tail agnostic policy in foldVMV_V_V (#105788)

This patch helps avoid regressions in an upcoming patch by making sure
we don't accidentally lose a tail agnostic policy when folding a vmv.v.v
into its source.

The previous comment about RISCVInsertVSETVLI relaxing the policy didn't
take into account the fact that there's a policy operand on vmv.v.v,
which can be tail agnostic.

If the tail is agnostic (via either the policy operand or the passthru
being undef) and vmv.v.v's VL <= Src's VL, then Src's tail can be made
agnostic.

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -529,10 +529,13 @@ bool RISCVVectorPeephole::foldVMV_V_V(MachineInstr &MI) {
                                               *Src->getParent()->getParent()));
   }
 
-  // Use a conservative tu,mu policy, RISCVInsertVSETVLI will relax it if
-  // passthru is undef.
-  Src->getOperand(RISCVII::getVecPolicyOpNum(Src->getDesc()))
-      .setImm(RISCVII::TAIL_UNDISTURBED_MASK_UNDISTURBED);
+  // If MI was tail agnostic and the VL didn't increase, preserve it.
+  int64_t Policy = RISCVII::TAIL_UNDISTURBED_MASK_UNDISTURBED;
+  bool TailAgnostic = (MI.getOperand(5).getImm() & RISCVII::TAIL_AGNOSTIC) ||
+                      Passthru.getReg() == RISCV::NoRegister;
+  if (TailAgnostic && isVLKnownLE(MI.getOperand(3), SrcVL))
+    Policy |= RISCVII::TAIL_AGNOSTIC;
+  Src->getOperand(RISCVII::getVecPolicyOpNum(Src->getDesc())).setImm(Policy);
 
   MRI->replaceRegWith(MI.getOperand(0).getReg(), Src->getOperand(0).getReg());
   MI.eraseFromParent();

llvm/test/CodeGen/RISCV/rvv/vmv.v.v-peephole.mir

@@ -18,3 +18,45 @@ body: |
     %y:gpr = ADDI $x0, 1
     %z:vr = PseudoVMV_V_V_M1 %passthru, %x, 4, 5 /* e32 */, 0 /* tu, mu */
 ...
+---
+name: tail_agnostic
+body: |
+  bb.0:
+    liveins: $v8
+    ; CHECK-LABEL: name: tail_agnostic
+    ; CHECK: liveins: $v8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %passthru:vr = COPY $v8
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 %passthru, $noreg, $noreg, 4, 5 /* e32 */, 1 /* ta, mu */
+    %passthru:vr = COPY $v8
+    %x:vr = PseudoVADD_VV_M1 %passthru, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
+    %y:vr = PseudoVMV_V_V_M1 %passthru, %x, 4, 5 /* e32 */, 1 /* ta, mu */
+...
+---
+name: tail_agnostic_larger_vl
+body: |
+  bb.0:
+    liveins: $v8
+    ; CHECK-LABEL: name: tail_agnostic_larger_vl
+    ; CHECK: liveins: $v8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %passthru:vr = COPY $v8
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 %passthru, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
+    %passthru:vr = COPY $v8
+    %x:vr = PseudoVADD_VV_M1 %passthru, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
+    %y:vr = PseudoVMV_V_V_M1 %passthru, %x, 5, 5 /* e32 */, 1 /* ta, mu */
+...
+---
+name: undef_passthru_src_undef_passthru
+body: |
+  bb.0:
+    liveins: $v8
+    ; CHECK-LABEL: name: undef_passthru_src_undef_passthru
+    ; CHECK: liveins: $v8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %passthru:vr = COPY $v8
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 4, 5 /* e32 */, 1 /* ta, mu */
+    %passthru:vr = COPY $v8
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
+    %y:vr = PseudoVMV_V_V_M1 $noreg, %x, 4, 5 /* e32 */, 0 /* tu, mu */
+...