[Bug target/68785] [6 Regression] valgrind reports issues with folding on x86_64

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68785

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
I believe this can happen when native_encode_expr encodes less than the
requested number of bytes (thus the program invokes undefined behavior).  In
this case
we read 4 bytes from "" (an empty string).

I'm placing some asserts to see what the program does and where.