add LLVM IR metadata to loads of pointers

[andrewrk]

http://llvm.org/docs/LangRef.html#load-instruction

The optional !nonnull metadata must reference a single metadata name corresponding to a metadata node with no entries. The existence of the !nonnull metadata on the instruction tells the optimizer that the value loaded is known to never be null. If the value is null at runtime, the behavior is undefined. This is analogous to the nonnull attribute on parameters and return values. This metadata can only be applied to loads of a pointer type.

The optional !dereferenceable metadata must reference a single metadata name <deref_bytes_node> corresponding to a metadata node with one i64 entry. The existence of the !dereferenceable metadata on the instruction tells the optimizer that the value loaded is known to be dereferenceable. The number of bytes known to be dereferenceable is specified by the integer value in the metadata node. This is analogous to the ‘’dereferenceable’’ attribute on parameters and return values. This metadata can only be applied to loads of a pointer type.

The optional !dereferenceable_or_null metadata must reference a single metadata name <deref_bytes_node> corresponding to a metadata node with one i64 entry. The existence of the !dereferenceable_or_null metadata on the instruction tells the optimizer that the value loaded is known to be either dereferenceable or null. The number of bytes known to be dereferenceable is specified by the integer value in the metadata node. This is analogous to the ‘’dereferenceable_or_null’’ attribute on parameters and return values. This metadata can only be applied to loads of a pointer type.

The optional !align metadata must reference a single metadata name <align_node> corresponding to a metadata node with one i64 entry. The existence of the !align metadata on the instruction tells the optimizer that the value loaded is known to be aligned to a boundary specified by the integer value in the metadata node. The alignment must be a power of 2. This is analogous to the ‘’align’’ attribute on parameters and return values. This metadata can only be applied to loads of a pointer type. If the returned value is not appropriately aligned at runtime, the behavior is undefined.

When we load pointers, we have all this information, so we should communicate it to LLVM.