[bolt] Fix std::prev()-past-begin in veneer handling code

[thakis]

Summary

matchLinkerVeneer() returns 3 if Instruction and the last
two instructions in [Instructions.begin, Instructions.end())
match the pattern

ADRP  x16, imm
ADD   x16, x16, imm
BR    x16

BinaryContext.cpp used to use

--Count;
for (auto It = std::prev(Instructions.end()); Count != 0;
     It = std::prev(It), --Count) {
  ...use It...
}

to walk these instructions. The first --Count skips the
instruction that's in Instruction instead of in Instructions.
The loop then walks over Instructions.

However, on the last iteration, this calls std::prev() on an
iterator that points at the container's begin(), which can blow
up.

Instead, use rbegin(), which sidesteps this issue.

Fixes test/AArch64/veneer-gold.s on a macOS host.
With this, check-bolt passes on macOS.

[thakis]

(ref D129260)

[yota9]

LGTM Thanks.

bolt/lib/Core/BinaryContext.cpp

auto matchVeneer = [&](BinaryFunction::InstrMapType &Instructions,
     const MCSymbol *VeneerSymbol = getOrCreateGlobalSymbol(Address, "FUNCat");
     BinaryFunction *Veneer = createBinaryFunction(VeneerSymbol->getName().str(),
                                                   *Section, Address, TotalSize);
     addAdrpAddRelocAArch64(*Veneer, *TargetLowBits, *TargetHiBits,
                            TargetAddress);
     MIB->addAnnotation(Instruction, "AArch64Veneer", true);
     Veneer->addInstruction(Offset, std::move(Instruction));
     --Count;
-    for (auto It = std::prev(Instructions.end()); Count != 0;
-         It = std::prev(It), --Count) {
+    for (auto It = Instructions.rbegin(); Count != 0; ++It, --Count) {
       MIB->addAnnotation(It->second, "AArch64Veneer", true);
       Veneer->addInstruction(It->first, std::move(It->second));
     }
 
     Veneer->getOrCreateLocalLabel(Address);
     Veneer->setMaxSize(TotalSize);
     Veneer->updateState(BinaryFunction::State::Disassembled);
     LLVM_DEBUG(dbgs() << "BOLT-DEBUG: handling veneer function at 0x" << Address