[DAG] Add SDPatternMatch::m_Load (#145481)

Add SDPatternMatch matcher and unit test coverage for `ISD::LOAD`
opcode.
This only matches the loaded value i.e. ResNo 0 and not the output
chain.
e.g.
```
m_Load(m_Value(), m_Value(), m_Value())
```

The first value is the input chain, the second is the base pointer, and
the last value is the offset.

Author: abhishek-kaushik22

llvm/include/llvm/CodeGen/SDPatternMatch.h

@@ -530,6 +530,13 @@ m_VSelect(const T0_P &Cond, const T1_P &T, const T2_P &F) {
   return TernaryOpc_match<T0_P, T1_P, T2_P>(ISD::VSELECT, Cond, T, F);
 }
 
+template <typename T0_P, typename T1_P, typename T2_P>
+inline Result_match<0, TernaryOpc_match<T0_P, T1_P, T2_P>>
+m_Load(const T0_P &Ch, const T1_P &Ptr, const T2_P &Offset) {
+  return m_Result<0>(
+      TernaryOpc_match<T0_P, T1_P, T2_P>(ISD::LOAD, Ch, Ptr, Offset));
+}
+
 template <typename T0_P, typename T1_P, typename T2_P>
 inline TernaryOpc_match<T0_P, T1_P, T2_P>
 m_InsertElt(const T0_P &Vec, const T1_P &Val, const T2_P &Idx) {

llvm/unittests/CodeGen/SelectionDAGPatternMatchTest.cpp

@@ -178,6 +178,12 @@ TEST_F(SelectionDAGPatternMatchTest, matchTernaryOp) {
   SDValue ExtractELT =
       DAG->getNode(ISD::EXTRACT_VECTOR_ELT, DL, Int32VT, V1, Op3);
 
+  SDValue Ch = DAG->getEntryNode();
+  SDValue BasePtr = DAG->getRegister(1, MVT::i64);
+  SDValue Offset = DAG->getUNDEF(MVT::i64);
+  MachinePointerInfo PtrInfo;
+  SDValue Load = DAG->getLoad(MVT::i32, DL, Ch, BasePtr, PtrInfo);
+
   using namespace SDPatternMatch;
   ISD::CondCode CC;
   EXPECT_TRUE(sd_match(ICMP_UGT, m_SetCC(m_Value(), m_Value(),
@@ -230,6 +236,13 @@ TEST_F(SelectionDAGPatternMatchTest, matchTernaryOp) {
   EXPECT_FALSE(sd_match(
       InsertSubvector,
       m_InsertSubvector(m_Specific(V2), m_Specific(V3), m_SpecificInt(3))));
+
+  EXPECT_TRUE(sd_match(
+      Load, m_Load(m_Specific(Ch), m_Specific(BasePtr), m_Specific(Offset))));
+
+  EXPECT_FALSE(
+      sd_match(Load.getValue(1), m_Load(m_Specific(Ch), m_Specific(BasePtr),
+                                        m_Specific(Offset))));
 }
 
 TEST_F(SelectionDAGPatternMatchTest, matchBinaryOp) {