RedundantLoadElimination: ignore memory effects of begin_access

Memory effects of begin_access are only defined to prevent the optimizer moving loads and stores across a begin_access.
But those memory effects are not relevant for RedundantLoadElimination

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/RedundantLoadElimination.swift

@@ -500,7 +500,7 @@ private struct InstructionScanner {
 
   private mutating func visit(instruction: Instruction) -> ScanResult {
     switch instruction {
-    case is FixLifetimeInst, is EndAccessInst, is EndBorrowInst:
+    case is FixLifetimeInst, is BeginAccessInst, is EndAccessInst, is EndBorrowInst:
       // Those scope-ending instructions are only irrelevant if the preceding load is not changed.
       // If it is changed from `load [copy]` -> `load [take]` the memory effects of those scope-ending
       // instructions prevent that the `load [take]` will illegally mutate memory which is protected

test/SILOptimizer/redundant_load_elim_ossa.sil

@@ -1403,6 +1403,24 @@ bb0:
   return %5 : $Int32
 }
 
+// CHECK-LABEL: sil [ossa] @ignore_begin_access :
+// CHECK:         return %0
+// CHECK-LABEL: } // end sil function 'ignore_begin_access'
+sil [ossa] @ignore_begin_access : $@convention(thin) (Int, Int, @inout TwoField) -> Int {
+bb0(%0 : $Int, %1 : $Int, %2 : $*TwoField):
+  %3 = begin_access [modify] [static] %2
+  %4 = struct_element_addr %3, #TwoField.a
+  store %0 to [trivial] %4
+  end_access %3
+  %7 = begin_access [modify] [static] %2
+  %8 = struct_element_addr %7, #TwoField.b
+  store %0 to [trivial] %8
+  end_access %7
+  %11 = struct_element_addr %2, #TwoField.a
+  %12 = load [trivial] %11
+  return %12
+}
+
 // CHECK: sil @tbaa_class_alias_nonclass
 // CHECK: strong_retain [[RET:%[0-9]+]]
 // CHECK: strong_retain [[RET]]