Remove the last remaining call to getPointerElementType() (halide#6715)

* Remove the last remaining call to getPointerElementType()

LLVM is moving to opaque pointers, we must have missed this one in previous work

* ARM vst mangling needs to be conditional on opaque ptrs

The fixes from last week regarding mangling of arm vst intrinsics needs to be made conditional on whether the pointer is opaque or not; this will change based on whether `-D CLANG_ENABLE_OPAQUE_POINTERS=ON|OFF` is defined when LLVM is built, but should be sniffed via this API, according to my LLVM contact.

* Revert "ARM vst mangling needs to be conditional on opaque ptrs"

This reverts commit 9901314.

src/CodeGen_X86.cpp

@@ -667,7 +667,7 @@ void CodeGen_X86::visit(const Load *op) {
         const Ramp *ramp = op->index.as<Ramp>();
         internal_assert(ramp) << "Expected AMXTile to have index ramp\n";
         Value *ptr = codegen_buffer_pointer(op->name, op->type, ramp->base);
-        LoadInst *load = builder->CreateAlignedLoad(ptr->getType()->getPointerElementType(), ptr, llvm::Align(op->type.bytes()));
+        LoadInst *load = builder->CreateAlignedLoad(llvm_type_of(upgrade_type_for_storage(op->type)), ptr, llvm::Align(op->type.bytes()));
         add_tbaa_metadata(load, op->name, op->index);
         value = load;
         return;