SIL: fix memory behavior of bind_memory and rebind_memory

Since dead-store-elimination is now more accurate, it's important that the `bind_memory` instructions are also defined to _read_ memory.
Otherwise they are not considered as barriers for dead-store-elimination.

Fixes a miscompile.

rdar://112150142

Author: eeckstein

include/swift/SIL/SILNodes.def

@@ -694,12 +694,12 @@ ABSTRACT_VALUE_AND_INST(SingleValueInstruction, ValueBase, SILInstruction)
                     SingleValueInstruction, MayHaveSideEffects, DoesNotRelease)
 
   // BindMemory and RebindMemory have no physical side effect. Semantically they
-  // write to their affected memory region because any reads or writes accessing
+  // access their affected memory region because any reads or writes accessing
   // that memory must be dependent on the bind operation.
   SINGLE_VALUE_INST(BindMemoryInst, bind_memory,
-                    SILInstruction, MayWrite, DoesNotRelease)
+                    SILInstruction, MayReadWrite, DoesNotRelease)
   SINGLE_VALUE_INST(RebindMemoryInst, rebind_memory,
-                    SILInstruction, MayWrite, DoesNotRelease)
+                    SILInstruction, MayReadWrite, DoesNotRelease)
 
 SINGLE_VALUE_INST_RANGE(SingleValueInstruction, AllocStackInst, RebindMemoryInst)
 NODE_RANGE(ValueBase, SILPhiArgument, RebindMemoryInst)

test/SILOptimizer/dead_store_elim.sil

@@ -1613,3 +1613,21 @@ bb0(%0 : $Int, %1 : @owned $IntAndFoo, %2 : $*IntAndFoo):
   %r = tuple ()
   return %r : $()
 }
+
+// CHECK-LABEL: sil @test_bind_memory  :
+// CHECK:         store %0
+// CHECK:       end sil function 'test_bind_memory'
+sil @test_bind_memory : $@convention(thin) (UInt64, Builtin.Word) -> Builtin.Int8 {
+bb0(%0 : $UInt64, %1 : $Builtin.Word):
+  %35 = alloc_stack $(UInt64, UInt64)
+  %36 = tuple_element_addr %35 : $*(UInt64, UInt64), 0
+  store %0 to %36 : $*UInt64
+  %50 = address_to_pointer %35 : $*(UInt64, UInt64) to $Builtin.RawPointer
+  %52 = bind_memory %50 : $Builtin.RawPointer, %1 : $Builtin.Word to $*UInt8
+  %72 = pointer_to_address %50 : $Builtin.RawPointer to [strict] $*UInt8
+  %75 = struct_element_addr %72 : $*UInt8, #UInt8._value
+  %76 = load %75 : $*Builtin.Int8
+  dealloc_stack %35 : $*(UInt64, UInt64)
+  return %76 : $Builtin.Int8
+}
+