AliasAnalysis: handle debug_step in computeMemoryEffect

The `debug_step` instruction does not have a memory effect on any specific address

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Analysis/AliasAnalysis.swift

@@ -220,7 +220,8 @@ struct AliasAnalysis {
          is StrongCopyUnmanagedValueInst,
          is StrongCopyWeakValueInst,
          is BeginBorrowInst,
-         is BeginCOWMutationInst:
+         is BeginCOWMutationInst,
+         is DebugStepInst:
       return .noEffects
 
     case let load as LoadInst:

SwiftCompilerSources/Sources/Optimizer/TestPasses/MemBehaviorDumper.swift

@@ -77,7 +77,8 @@ private extension Instruction {
          is StoreBorrowInst,
          is MarkDependenceInst,
          is MarkDependenceAddrInst,
-         is DebugValueInst:
+         is DebugValueInst,
+         is DebugStepInst:
       return true
     default:
       return false

test/SILOptimizer/mem-behavior.sil

@@ -1266,6 +1266,18 @@ bb0:
   return %r : $()
 }
 
+// CHECK-LABEL: @test_debug_step
+// CHECK:       PAIR #0.
+// CHECK-NEXT:     debug_step
+// CHECK-NEXT:     %0 = argument of bb0
+// CHECK-NEXT:     r=0,w=0
+sil [ossa] @test_debug_step : $@convention(thin) (@inout_aliasable Int) -> () {
+bb0(%0 : $*Int):
+  debug_step
+  %99 = tuple ()
+  return %99 : $()
+}
+
 // CHECK-LABEL: @test_store_assign
 // CHECK:       PAIR #0.
 // CHECK-NEXT:      store %2 to [assign] %0 : $*C