regress: Don't provoke undefined behaviour
The 3 - 4 in this expression is evaluated before the left shift
(as far as I understand it this is standardized, but unexpected, and
clang-14 warns about it). The resulting left shift by a negative
number is undefined behaviour (both gcc-13 and clang-14 warn about this,
and produce arbitrary results; gcc's seems deterministic between
program runs but clang's is not).
All that's needed here is a somewhat complicated expression, so change it to avoid UB.
Resolves: #458 (closed)