[BOLT] Use opcode name in hashBlock

[Amir]

Summary

Use MCInst opcode name instead of opcode value in hashing.

Opcode values are unstable wrt changes to target tablegen definitions,
and we notice that as output mismatches in NFC testing. This makes BOLT YAML
profile tied to a particular LLVM revision which is less portable than
offset-based fdata profile.

Switch to using opcode names which have 1:1 mapping with opcode values for any
given LLVM revision, and are stable wrt modifications to .td files (except of
course modifications to names themselves).

Test Plan:
D150154 is a test commit adding new X86 instruction which shifts opcode values.
With current change, pre-aggregated-perf.test passes in nfc check mode.
Without current change, pre-aggregated-perf.test expectedly fails.

[rafauler]

Thanks

bolt/lib/Core/HashUtilities.cpp

 //===- bolt/Core/HashUtilities.cpp - Misc hash utilities ------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Computation of hash values over BinaryFunction and BinaryBasicBlock.
 //
 //===----------------------------------------------------------------------===//
 
 #include "bolt/Core/HashUtilities.h"
 #include "bolt/Core/BinaryContext.h"
 #include "bolt/Core/BinaryFunction.h"
+#include "llvm/MC/MCInstPrinter.h"
 
 namespace llvm {
 namespace bolt {
 
 /// Hashing a 64-bit integer to a 16-bit one.
 uint16_t hash_64_to_16(const uint64_t Hash) {
   uint16_t Res = (uint16_t)(Hash & 0xFFFF);
   Res ^= (uint16_t)((Hash >> 16) & 0xFFFF);
for (const MCInst &Inst : BB) {
     // basic blocks in order and do not rely on branches to be in-sync with
     // CFG. Note that we still use condition code of conditional jumps.
     if (BC.MIB->isUnconditionalBranch(Inst))
       continue;
 
     if (IsX86 && BC.MIB->isConditionalBranch(Inst))
       Opcode = BC.MIB->getShortBranchOpcode(Opcode);
 
-    if (Opcode == 0)
+    if (Opcode == 0) {
       HashString.push_back(0);
-
-    while (Opcode) {
-      uint8_t LSB = Opcode & 0xff;
-      HashString.push_back(LSB);
-      Opcode = Opcode >> 8;
+    } else {
+      StringRef OpcodeName = BC.InstPrinter->getOpcodeName(Opcode);
+      HashString.append(OpcodeName.str());
     }
 
     for (const MCOperand &Op : MCPlus::primeOperands(Inst))
       HashString.append(OperandHashFunc(Op));
   }
   return HashString;
 }
 
 } // namespace bolt
 } // namespace llvm