[LVA] Support access instructions in DCE.

Simple check for the following pattern:
x = begin_access
end_access x

Author: zoecarver

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -40,6 +40,12 @@ namespace {
 // FIXME: Reconcile the similarities between this and
 //        isInstructionTriviallyDead.
 static bool seemsUseful(SILInstruction *I) {
+  if (auto access = dyn_cast<BeginAccessInst>(I))
+    return access->getSingleUse() == nullptr && !access->use_empty();
+  
+  if (isa<EndAccessInst>(I))
+    return I->getOperand(0)->getSingleUse() == nullptr;
+  
   if (I->mayHaveSideEffects())
     return true;
 

test/SILOptimizer/dead_code_elimination.sil

@@ -259,3 +259,21 @@ bb2:
 bb3:
   br bb1
 }
+
+// Check that DCE eliminates dead access instructions.
+// CHECK-LABEL: sil @dead_access
+// CHECK: bb0
+// CHECK-NEXT: tuple
+// CHECK-NEXT: return
+// CHECK-LABEL: end sil function 'dead_access'
+sil @dead_access : $@convention(thin) (@in Container) -> () {
+bb0(%0 : $*Container):
+  %1 = begin_access [modify] [dynamic] %0 : $*Container
+  end_access %1 : $*Container
+  
+  %3 = begin_access [read] [static] %0 : $*Container
+  end_access %3 : $*Container
+
+  %999 = tuple ()
+  return %999 : $()
+}