Previously, this pass would convert any read-only use of a
runtime.stringToBytes call to use the original string buffer instead.
This is incorrect: if there are any writes to the resulting buffer, none
of the slice buffer pointers can be converted to use the original
read-only string buffer.
This commit fixes that bug and adds a test to prove the new (correct)
behavior.
Unfortunately, while doing this I found that it doesn't actually apply
in any real-world programs (tested with `make smoketest`), apparently
because nil pointer checking messes with the functionattrs pass. I hope
to fix that after moving to LLVM 9, which has an optimization that makes
nil pointer checking easier to implement.
