IRGen: ensure ordering of instructions (NFC)

Split out the subconstructors to ensure guaranteed IR ordering irrespective of
order of evaluation of parameters which is not specified.  This is needed for
identical IRGen on Windows.  NFC.

Author: compnerd

lib/IRGen/GenExistential.cpp

@@ -2108,10 +2108,9 @@ static llvm::Constant *getDeallocateBoxedOpaqueExistentialBufferFunction(
         //  Size = ((sizeof(HeapObject) + align) & ~align) + size
         auto *heapHeaderSize = llvm::ConstantInt::get(
             IGF.IGM.SizeTy, IGM.RefCountedStructSize.getValue());
-        size = Builder.CreateAdd(
-            Builder.CreateAnd(Builder.CreateAdd(heapHeaderSize, alignmentMask),
-                              Builder.CreateNot(alignmentMask)),
-            size);
+        auto *Add = Builder.CreateAdd(heapHeaderSize, alignmentMask);
+        auto *Not = Builder.CreateNot(alignmentMask);
+        size = Builder.CreateAdd(Builder.CreateAnd(Add, Not), size);
 
         // At least pointer aligned.
         //  AlignmentMask = alignmentMask | alignof(void*) - 1
@@ -2202,9 +2201,9 @@ getProjectBoxedOpaqueExistentialFunction(IRGenFunction &IGF,
           //  StartOffset = ((sizeof(HeapObject) + align) & ~align)
           auto *heapHeaderSize = llvm::ConstantInt::get(
               IGF.IGM.SizeTy, IGM.RefCountedStructSize.getValue());
-          auto *startOffset = Builder.CreateAnd(
-              Builder.CreateAdd(heapHeaderSize, alignmentMask),
-              Builder.CreateNot(alignmentMask));
+          auto *Add = Builder.CreateAdd(heapHeaderSize, alignmentMask);
+          auto *Not = Builder.CreateNot(alignmentMask);
+          auto *startOffset = Builder.CreateAnd(Add, Not);
           auto *addressInBox =
               IGF.emitByteOffsetGEP(boxReference, startOffset, IGM.OpaqueTy);
           IGF.Builder.CreateRet(addressInBox);