Fix dereferencing in overrides with llvm_assert_eq
Fix dereferencing in overrides with llvm_assert_eq

Pointer assignments in llvm_assert_eq and related commands, which
previously weren't supported, were being treated incorrectly. The
introduction of llvm_assert_null exposed this problem, and this commit
removes it.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script