[BOLT][AArch64] Skip as many as possible zero's in code padding validation

We are skipping four zero's as a time when validating code padding in case
that the next zero would be part of an instruction or constant island, and
for functions that have large amount of padding (like due to hugify), this
could be very slow. We now change the validation to skip as many as possible
but need to be 4's exact multiple number of zero's. No valid instruction has
encoding as 0x00000000 and even if we stumble into some constant island, the
API `BinaryFunction::isInConstantIsland()` has been made to find the size
between the asked address and the end of island (llvm#164037), so this should
be safe.

Author: yozhu

bolt/lib/Core/BinaryContext.cpp

@@ -1010,14 +1010,12 @@ bool BinaryContext::hasValidCodePadding(const BinaryFunction &BF) {
     return Offset - StartOffset;
   };
 
-  // Skip a sequence of zero bytes. For AArch64 we only skip 4 bytes of zeros
-  // in case the following zeros belong to constant island or veneer.
+  // Skip a sequence of zero bytes. For AArch64 we only skip 4's exact
+  // multiple number of zeros in case the following zeros belong to veneer.
   auto skipZeros = [&]() {
     const uint64_t StartOffset = Offset;
     uint64_t CurrentOffset = Offset;
-    for (; CurrentOffset < BF.getMaxSize() &&
-           (!isAArch64() || CurrentOffset < StartOffset + 4);
-         ++CurrentOffset)
+    for (; CurrentOffset < BF.getMaxSize(); ++CurrentOffset)
       if ((*FunctionData)[CurrentOffset] != 0)
         break;