[IRGen] Fix FixedArray of fixedSize 1 elt addring.

When a `FixedArray`'s fixed size is 1, it looks like `[1 x %Ty]`. Given
an array's address, performing an operation on each element's address
entail's indexing into the array to each element's index to produce an
element's address for each index.  That is true even when the array
consists of a single element.  In that case, produce an address for that
single element by indexing to index 0 into each passed-in array.

rdar://151726387

Author: nate-chandler

lib/IRGen/GenArray.cpp

@@ -53,8 +53,15 @@ class ArrayTypeInfoBase : public BaseTypeInfo {
       return;
     }
     if (fixedSize == 1) {
-      // only one element to operate on
-      return body(addrs);
+      auto zero = llvm::ConstantInt::get(IGF.IGM.IntPtrTy, 0);
+      // only one element to operate on; index to it in each array
+      SmallVector<Address, 2> eltAddrs;
+      eltAddrs.reserve(addrs.size());
+      for (auto index : indices(addrs)) {
+        eltAddrs.push_back(Element.indexArray(IGF, addrs[index], zero,
+                                              getElementSILType(IGF.IGM, T)));
+      }
+      return body(eltAddrs);
     }
 
     auto arraySize = getArraySize(IGF, T);

validation-test/IRGen/rdar151726387.sil

@@ -0,0 +1,21 @@
+// RUN: %target-swift-frontend -parse-sil -emit-ir %s
+
+sil_stage canonical
+
+import Builtin
+import Swift
+
+struct O {
+  var arr: Builtin.FixedArray<1, I>
+  struct I {
+    var eyes: SIMD64<Int8>
+    var neighs: String
+  }
+}
+
+sil @copy : $@convention(thin) (@in_guaranteed O) -> (@out O) {
+entry(%out : $*O, %in : $*O):
+  copy_addr %in to [init] %out : $*O
+  %retval = tuple ()
+  return %retval : $()
+}