Support zero size types in StackSafetyAnalysis.

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Build artifacts: diff.json, console-log.txt

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llvm/lib/Analysis/StackSafetyAnalysis.cpp

StackSafetyLocalAnalysis::offsetFromAlloca(Value *Addr,
   ConstantRange Offset = SE.getUnsignedRange(Expr).zextOrTrunc(PointerSize);
   assert(!Offset.isEmptySet());
   return Offset;
 }
 
 ConstantRange
 StackSafetyLocalAnalysis::getAccessRange(Value *Addr, const Value *AllocaPtr,
                                          ConstantRange SizeRange) {
+  // Zero-size loads and stores do not access memory.
+  if (SizeRange.isEmptySet())
+    return ConstantRange::getEmpty(PointerSize);
+
   if (!SE.isSCEVable(Addr->getType()))
     return UnknownRange;
 
   AllocaOffsetRewriter Rewriter(SE, AllocaPtr);
   const SCEV *Expr = Rewriter.visit(SE.getSCEV(Addr));
 
   ConstantRange AccessStartRange =
       SE.getUnsignedRange(Expr).zextOrTrunc(PointerSize);

llvm/test/Analysis/StackSafetyAnalysis/local.ll

 ; CHECK-NOT: ]:
 entry:
   %x = alloca <vscale x 4 x i32>, align 4
   %x1 = bitcast <vscale x 4 x i32>* %x to i8*
   store i8 0, i8* %x1, align 1
   store <vscale x 4 x i32> %v, <vscale x 4 x i32>* %p, align 4
   ret void
 }
+
+%zerosize_type = type {}
+
+define void @ZeroSize(%zerosize_type *%p)  {
+; CHECK-LABEL: @ZeroSize dso_preemptable{{$}}
+; CHECK-NEXT: args uses:
+; CHECK-NEXT:   p[]: empty-set
+; CHECK-NEXT: allocas uses:
+; CHECK-NEXT:   x[0]: empty-set
+; CHECK-NOT: ]:
+entry:
+  %x = alloca %zerosize_type, align 4
+  store %zerosize_type undef, %zerosize_type* %x, align 4
+  store %zerosize_type undef, %zerosize_type* undef, align 4
+  %val = load %zerosize_type, %zerosize_type* %p, align 4
+  ret void
+}