LLVM: Use getelementptr instruction for a lot of situations

LLVM: Use getelementptr instruction for a lot of situations

LLVM supports creating pointers in two ways, firstly through
pointer arithmetic (by casting between pointers and ints)
and secondly using the getelementptr instruction. The second way
is preferable as it gives LLVM more information to work with.

This patch changes a lot of pointer related code from the first
method to the getelementptr method.