[NFC][AArch64] Handle processLogicalImmediate error

If processLogicalImmediate fails, we should return from the function
without changing InsInstrs or DelInstrs. This happens for
CodeGen/AArch64/urem-seteq-nonzero.ll LIT test as described in
https://reviews.llvm.org/D99662#2662296.

Callers of genAlternativeCodeSequence skip patterns where InsInstrs
stays empty, so this does not cause any issues now.

Differential Revision: https://reviews.llvm.org/D100047

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -5432,15 +5432,15 @@ void AArch64InstrInfo::genAlternativeCodeSequence(
     }
     uint64_t UImm = SignExtend64(Imm, BitSize);
     uint64_t Encoding;
-    if (AArch64_AM::processLogicalImmediate(UImm, BitSize, Encoding)) {
-      MachineInstrBuilder MIB1 =
-          BuildMI(MF, Root.getDebugLoc(), TII->get(OrrOpc), NewVR)
-              .addReg(ZeroReg)
-              .addImm(Encoding);
-      InsInstrs.push_back(MIB1);
-      InstrIdxForVirtReg.insert(std::make_pair(NewVR, 0));
-      MUL = genMaddR(MF, MRI, TII, Root, InsInstrs, 1, Opc, NewVR, RC);
-    }
+    if (!AArch64_AM::processLogicalImmediate(UImm, BitSize, Encoding))
+      return;
+    MachineInstrBuilder MIB1 =
+        BuildMI(MF, Root.getDebugLoc(), TII->get(OrrOpc), NewVR)
+            .addReg(ZeroReg)
+            .addImm(Encoding);
+    InsInstrs.push_back(MIB1);
+    InstrIdxForVirtReg.insert(std::make_pair(NewVR, 0));
+    MUL = genMaddR(MF, MRI, TII, Root, InsInstrs, 1, Opc, NewVR, RC);
     break;
   }
   case MachineCombinerPattern::MULSUBW_OP1:
@@ -5523,15 +5523,15 @@ void AArch64InstrInfo::genAlternativeCodeSequence(
     }
     uint64_t UImm = SignExtend64(-Imm, BitSize);
     uint64_t Encoding;
-    if (AArch64_AM::processLogicalImmediate(UImm, BitSize, Encoding)) {
-      MachineInstrBuilder MIB1 =
-          BuildMI(MF, Root.getDebugLoc(), TII->get(OrrOpc), NewVR)
-              .addReg(ZeroReg)
-              .addImm(Encoding);
-      InsInstrs.push_back(MIB1);
-      InstrIdxForVirtReg.insert(std::make_pair(NewVR, 0));
-      MUL = genMaddR(MF, MRI, TII, Root, InsInstrs, 1, Opc, NewVR, RC);
-    }
+    if (!AArch64_AM::processLogicalImmediate(UImm, BitSize, Encoding))
+      return;
+    MachineInstrBuilder MIB1 =
+        BuildMI(MF, Root.getDebugLoc(), TII->get(OrrOpc), NewVR)
+            .addReg(ZeroReg)
+            .addImm(Encoding);
+    InsInstrs.push_back(MIB1);
+    InstrIdxForVirtReg.insert(std::make_pair(NewVR, 0));
+    MUL = genMaddR(MF, MRI, TII, Root, InsInstrs, 1, Opc, NewVR, RC);
     break;
   }
 
@@ -6221,9 +6221,6 @@ void AArch64InstrInfo::genAlternativeCodeSequence(
   }
   } // end switch (Pattern)
   // Record MUL and ADD/SUB for deletion
-  // FIXME: This assertion fails in CodeGen/AArch64/tailmerging_in_mbp.ll and
-  // CodeGen/AArch64/urem-seteq-nonzero.ll.
-  // assert(MUL && "MUL was never set");
   if (MUL)
     DelInstrs.push_back(MUL);
   DelInstrs.push_back(&Root);