EvalStore/EvalLoad to handle all loads/stores from symbolic memory, allowing us
to do checks for null dereferences, etc., at any arbitrary load/store (these
were missed checks before). This also resulted in some major cleanups, some
conceptual, and others just in the structure of the code.

This temporarily introduces a regression in the test suite (null-deref-ps.c)
before I add a new LVal type for structure fields.

llvm-svn: 50443