[SVE][CodeGen] Replace use of TypeSize operator< in GlobalMerge::doMerge

[david-arm]

Summary

We don't support global variables with scalable vector types so I've
changed the code to compare the fixed sizes instead.

[efriedma]

LGTM

llvm/lib/CodeGen/GlobalMerge.cpp

 INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "Merge global variables", false, false)
 
 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                           Module &M, bool isConst, unsigned AddrSpace) const {
   auto &DL = M.getDataLayout();
   // FIXME: Find better heuristics
   llvm::stable_sort(
       Globals, [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
-        return DL.getTypeAllocSize(GV1->getValueType()) <
-               DL.getTypeAllocSize(GV2->getValueType());
+        // We don't support scalable global variables.
+        return DL.getTypeAllocSize(GV1->getValueType()).getFixedSize() <
+               DL.getTypeAllocSize(GV2->getValueType()).getFixedSize();
       });
 
   // If we want to just blindly group all globals together, do so.
   if (!GlobalMergeGroupByUse) {
     BitVector AllGlobals(Globals.size());
     AllGlobals.set();
     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
   }