[GlobalISel] Add a m_SpecificReg matcher

Similar to the specific matchers for constants.

The intention here is to make it easier to write combines which check if a
specific register is used more than once.

e.g. matching patterns like:

```
(X + Y) == Y
```

Differential Revision: https://reviews.llvm.org/D135378

llvm/include/llvm/CodeGen/GlobalISel/MIPatternMatch.h

@@ -237,6 +237,20 @@ inline SpecificConstantMatch m_AllOnesInt() {
 }
 ///}
 
+/// Matcher for a specific register.
+struct SpecificRegisterMatch {
+  Register RequestedReg;
+  SpecificRegisterMatch(Register RequestedReg) : RequestedReg(RequestedReg) {}
+  bool match(const MachineRegisterInfo &MRI, Register Reg) {
+    return Reg == RequestedReg;
+  }
+};
+
+/// Matches a register only if it is equal to \p RequestedReg.
+inline SpecificRegisterMatch m_SpecificReg(Register RequestedReg) {
+  return SpecificRegisterMatch(RequestedReg);
+}
+
 // TODO: Rework this for different kinds of MachineOperand.
 // Currently assumes the Src for a match is a register.
 // We might want to support taking in some MachineOperands and call getReg on

llvm/unittests/CodeGen/GlobalISel/PatternMatchTest.cpp

@@ -769,6 +769,23 @@ TEST_F(AArch64GISelMITest, MatchNot) {
   EXPECT_TRUE(mi_match(AddInst.getReg(2), *MRI, m_Not(m_Reg(NotReg))));
   EXPECT_EQ(NotReg, Copies[0]);
 }
+
+TEST_F(AArch64GISelMITest, MatchSpecificReg) {
+  setUp();
+  if (!TM)
+    return;
+  auto Cst1 = B.buildConstant(LLT::scalar(64), 42);
+  auto Cst2 = B.buildConstant(LLT::scalar(64), 314);
+  Register Reg = Cst1.getReg(0);
+  // Basic case: Same register twice.
+  EXPECT_TRUE(mi_match(Reg, *MRI, m_SpecificReg(Reg)));
+  // Basic case: Two explicitly different registers.
+  EXPECT_FALSE(mi_match(Reg, *MRI, m_SpecificReg(Cst2.getReg(0))));
+  // Check that we can tell that an instruction uses a specific register.
+  auto Add = B.buildAdd(LLT::scalar(64), Cst1, Cst2);
+  EXPECT_TRUE(mi_match(Add.getReg(0), *MRI, m_GAdd(m_SpecificReg(Reg), m_Reg())));
+}
+
 } // namespace
 
 int main(int argc, char **argv) {