Bugfix: add field-insensitive information upon reaching a field-sensitive store

[jordr]

• 88b2a38 fixes the reduced small.c breaking example from https://gist.github.com/jordr/55817398f88255dedfe206dd8337dba1
• 1eb3f6f adds small.c as a unit test at test/Slicing/field-sensitivity-hetero.c. Without 88b2a38, this unit test fails
• this does not fix the original posix example from https://gist.github.com/jordr/55817398f88255dedfe206dd8337dba1