add support for memcpy/memset with opaque pointers

[rjodinchr]

Use InferType to get the type. If InferType returns null, it means that we are not capable of inferring the type. In that case, choose an appropriate type and add a gep so that future pass can infer the type we choose. ReplacePointerBitcast might rework what we did in this pass to have something coherent at a function level.

Fix memcpy, which did not work as expected when having no unpacking but a type smaller than the size of the copy. Fix it by removing the if statement which is not needed. It will produce unnecessary gep, which is not a issue.

Fix memset to work on a subset of a type. It still needs to be complete type, like for memcpy.