[NFC][InstrInfoEmitter] Include location of inst definition in comment (#156927)

Print the source location of the instruction definition in comment next
to the enum value for each instruction. To make this more readable,
change formatting of the instruction enums to be better aligned.

Example output:

```
    VLD4qWB_register_Asm_8                 = 573, // (ARMInstrNEON.td:8849)
    VMOVD0                                 = 574, // (ARMInstrNEON.td:6337)
    VMOVDcc                                = 575, // (ARMInstrVFP.td:2466)
    VMOVHcc                                = 576, // (ARMInstrVFP.td:2474)
    VMOVQ0                                 = 577, // (ARMInstrNEON.td:6341)
```

Author: jurahul

llvm/utils/TableGen/Common/CodeGenInstruction.h

@@ -320,6 +320,8 @@ class CodeGenInstruction {
     return RV && isa<DagInit>(RV->getValue());
   }
 
+  StringRef getName() const { return TheDef->getName(); }
+
 private:
   bool isOperandImpl(StringRef OpListName, unsigned i,
                      StringRef PropertyName) const;

llvm/utils/TableGen/InstrInfoEmitter.cpp

@@ -25,6 +25,8 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/Format.h"
+#include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Record.h"
@@ -1302,18 +1304,28 @@ void InstrInfoEmitter::emitEnums(
 
   OS << "namespace llvm::" << Namespace << " {\n";
 
+  auto II = llvm::max_element(
+      NumberedInstructions,
+      [](const CodeGenInstruction *InstA, const CodeGenInstruction *InstB) {
+        return InstA->getName().size() < InstB->getName().size();
+      });
+  size_t MaxNameSize = (*II)->getName().size();
+
   OS << "  enum {\n";
-  for (const CodeGenInstruction *Inst : NumberedInstructions)
-    OS << "    " << Inst->TheDef->getName()
-       << "\t= " << Target.getInstrIntValue(Inst->TheDef) << ",\n";
+  for (const CodeGenInstruction *Inst : NumberedInstructions) {
+    OS << "    " << left_justify(Inst->TheDef->getName(), MaxNameSize) << " = "
+       << Target.getInstrIntValue(Inst->TheDef) << ", // "
+       << SrcMgr.getFormattedLocationNoOffset(Inst->TheDef->getLoc().front())
+       << '\n';
+  }
   OS << "    INSTRUCTION_LIST_END = " << NumberedInstructions.size() << '\n';
-  OS << "  };\n\n";
+  OS << "  };\n";
   OS << "} // end namespace llvm::" << Namespace << '\n';
   OS << "#endif // GET_INSTRINFO_ENUM\n\n";
 
   OS << "#ifdef GET_INSTRINFO_SCHED_ENUM\n";
   OS << "#undef GET_INSTRINFO_SCHED_ENUM\n";
-  OS << "namespace llvm::" << Namespace << "::Sched {\n\n";
+  OS << "namespace llvm::" << Namespace << "::Sched {\n";
   OS << "  enum {\n";
   auto ExplictClasses = SchedModels.explicitSchedClasses();
   for (const auto &[Idx, Class] : enumerate(ExplictClasses))