[BPF] Use elementtype attribute for preserve.array/struct.index intrinsics

Use the elementtype attribute introduced in D105407 for the
llvm.preserve.array/struct.index intrinsics. It carries the
element type of the GEP these intrinsics effectively encode.

This patch:

 * Adds a verifier check that the attribute is required.
 * Adds it in the IRBuilder methods for these intrinsics.
 * Autoupgrades old bitcode without the attribute.
 * Updates the lowering code to use the attribute rather than
   the pointer element type.
 * Updates lots of tests to specify the attribute.
 * Adds -force-opaque-pointers to the intrinsic-array.ll test
   to demonstrate they work now.

https://reviews.llvm.org/D106184
diff --git a/llvm/test/Bitcode/upgrade-elementtype.ll b/llvm/test/Bitcode/upgrade-elementtype.ll
new file mode 100644
index 0000000..16c3605
--- /dev/null
+++ b/llvm/test/Bitcode/upgrade-elementtype.ll
@@ -0,0 +1,15 @@
+; RUN: opt -S < %s.bc | FileCheck %s
+
+%struct.s = type { i32, i32 }
+
+define void @test(%struct.s* %arg) {
+; CHECK-LABEL: define void @test
+; CHECK: %x = call %struct.s* @llvm.preserve.array.access.index.p0s_struct.ss.p0s_struct.ss(%struct.s* elementtype(%struct.s) %arg, i32 0, i32 2)
+; CHECK: %1 = call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.ss(%struct.s* elementtype(%struct.s) %x, i32 1, i32 1)
+  %x = call %struct.s* @llvm.preserve.array.access.index.p0s_struct.ss.p0s_struct.ss(%struct.s* %arg, i32 0, i32 2)
+  call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.ss(%struct.s* %x, i32 1, i32 1)
+  ret void
+}
+
+declare %struct.s* @llvm.preserve.array.access.index.p0s_struct.ss.p0s_struct.ss(%struct.s*, i32, i32)
+declare i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.ss(%struct.s*, i32, i32)