Set the default value for MaxAtomicSizeInBitsSupported to 0.

This was planned since its introduction, but wasn't rolled out for a
little bit longer than intended (ahem...8 years).

All in-tree targets have now been adjusted to call
setMaxAtomicSizeInBitsSupported explicitly where required, so this
should be a no-op. The docs in docs/Atomics.rst already claimed the
default was 0, so that doesn't need updating.

Author: jyknight

llvm/lib/CodeGen/TargetLoweringBase.cpp

@@ -752,9 +752,7 @@ TargetLoweringBase::TargetLoweringBase(const TargetMachine &tm) : TM(tm) {
   GatherAllAliasesMaxDepth = 18;
   IsStrictFPEnabled = DisableStrictNodeMutation;
   MaxBytesForAlignment = 0;
-  // TODO: the default will be switched to 0 in the next commit, along
-  // with the Target-specific changes necessary.
-  MaxAtomicSizeInBitsSupported = 1024;
+  MaxAtomicSizeInBitsSupported = 0;
 
   // Assume that even with libcalls, no target supports wider than 128 bit
   // division.