[AArch64][GlobalISel] Optimize conjunctions of compares to conditional compares.

This is a partial port of the same optimization from AArch64ISelLowering,
although the original handles more cases when generating regular compares
instead of this one which just does it when selecting G_SELECTs.

For more detailed comments see the original comments for
emitConditionalComparison() in AArch64ISelLowering.

Gives minor code size improvements.

Differential Revision: https://reviews.llvm.org/D117166

Author: aemerson

llvm/include/llvm/CodeGen/GlobalISel/GenericMachineInstrs.h

@@ -14,6 +14,7 @@
 #ifndef LLVM_CODEGEN_GLOBALISEL_GENERICMACHINEINSTRS_H
 #define LLVM_CODEGEN_GLOBALISEL_GENERICMACHINEINSTRS_H
 
+#include "llvm/IR/Instructions.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/TargetOpcodes.h"
@@ -226,6 +227,37 @@ class GSelect : public GenericMachineInstr {
   }
 };
 
+/// Represent a G_ICMP or G_FCMP.
+class GAnyCmp : public GenericMachineInstr {
+public:
+  CmpInst::Predicate getCond() const {
+    return static_cast<CmpInst::Predicate>(getOperand(1).getPredicate());
+  }
+  Register getLHSReg() const { return getReg(2); }
+  Register getRHSReg() const { return getReg(3); }
+
+  static bool classof(const MachineInstr *MI) {
+    return MI->getOpcode() == TargetOpcode::G_ICMP ||
+           MI->getOpcode() == TargetOpcode::G_FCMP;
+  }
+};
+
+/// Represent a G_ICMP.
+class GICmp : public GAnyCmp {
+public:
+  static bool classof(const MachineInstr *MI) {
+    return MI->getOpcode() == TargetOpcode::G_ICMP;
+  }
+};
+
+/// Represent a G_FCMP.
+class GFCmp : public GAnyCmp {
+public:
+  static bool classof(const MachineInstr *MI) {
+    return MI->getOpcode() == TargetOpcode::G_FCMP;
+  }
+};
+
 } // namespace llvm
 
 #endif // LLVM_CODEGEN_GLOBALISEL_GENERICMACHINEINSTRS_H