[RISCV] Use vadd.vi for tail undisturbe vsub intrinsic with small immediate.

Our pattern previously checked for the merge operand being undef.

Author: topperc

llvm/lib/Target/RISCV/RISCVInstrInfoVPseudos.td

@@ -6297,15 +6297,16 @@ foreach vti = AllIntegerVectors in {
                                                         (XLenVT timm:$policy))>;
 
     // Match VSUB with a small immediate to vadd.vi by negating the immediate.
-    def : Pat<(vti.Vector (int_riscv_vsub (vti.Vector (undef)),
+    def : Pat<(vti.Vector (int_riscv_vsub (vti.Vector vti.RegClass:$merge),
                                           (vti.Vector vti.RegClass:$rs1),
                                           (vti.Scalar simm5_plus1:$rs2),
                                           VLOpFrag)),
-              (!cast<Instruction>("PseudoVADD_VI_"#vti.LMul.MX) (vti.Vector (IMPLICIT_DEF)),
-                                                                vti.RegClass:$rs1,
-                                                                (NegImm simm5_plus1:$rs2),
-                                                                GPR:$vl,
-                                                                vti.Log2SEW, TA_MA)>;
+              (!cast<Instruction>("PseudoVADD_VI_"#vti.LMul.MX)
+                                                      vti.RegClass:$merge,
+                                                      vti.RegClass:$rs1,
+                                                      (NegImm simm5_plus1:$rs2),
+                                                      GPR:$vl,
+                                                      vti.Log2SEW, TA_MA)>;
     def : Pat<(vti.Vector (int_riscv_vsub_mask (vti.Vector vti.RegClass:$merge),
                                                (vti.Vector vti.RegClass:$rs1),
                                                (vti.Scalar simm5_plus1:$rs2),