[SDPatternMatch] Add m_Poison matcher (#144860)

Add SDPatternMatch matcher and unit test coverage for ISD::POISON opcode

e.g.
```
m_InsertElt(m_Poison(), m_Value(), m_Zero())
```

Author: NexMing

llvm/include/llvm/CodeGen/SDPatternMatch.h

@@ -142,6 +142,8 @@ inline Opcode_match m_Opc(unsigned Opcode) { return Opcode_match(Opcode); }
 
 inline Opcode_match m_Undef() { return Opcode_match(ISD::UNDEF); }
 
+inline Opcode_match m_Poison() { return Opcode_match(ISD::POISON); }
+
 template <unsigned NumUses, typename Pattern> struct NUses_match {
   Pattern P;
 

llvm/unittests/CodeGen/SelectionDAGPatternMatchTest.cpp

@@ -493,6 +493,11 @@ TEST_F(SelectionDAGPatternMatchTest, matchConstants) {
   SDValue UndefVInt32VT = DAG->getUNDEF(VInt32VT);
   EXPECT_TRUE(sd_match(UndefInt32VT, m_Undef()));
   EXPECT_TRUE(sd_match(UndefVInt32VT, m_Undef()));
+
+  SDValue PoisonInt32VT = DAG->getPOISON(Int32VT);
+  SDValue PoisonVInt32VT = DAG->getPOISON(VInt32VT);
+  EXPECT_TRUE(sd_match(PoisonInt32VT, m_Poison()));
+  EXPECT_TRUE(sd_match(PoisonVInt32VT, m_Poison()));
 }
 
 TEST_F(SelectionDAGPatternMatchTest, patternCombinators) {