Read the limit of a C loop on every pass when the loop counts down

[j-modernc-org]

This is the fix suggested in totalspectrum/flexprop#119, limited to C.

CheckSimpleIncrementLoop rewrites for (i = 0; i < n; i++), with i not used in the body, as a count down from n. That reads n once, before the loop. In C the limit is read on every pass, so a local the body changes, a global, and a global that a called function changes all ran the count they had when the loop began.

For C, the rewrite now requires a limit that cannot change in the loop: a constant, or a local whose address is not taken and which the body does not modify, or a sum or difference of those. A loop whose limit is such a local is still counted down. Spin and BASIC loops are rewritten as before, since reading the limit once may be what those languages mean.

Test/cexec07.c has the three loops from the issue and one whose limit the body leaves alone. It prints 5 5 5 5 on master and 3 3 3 5 with this change, as gcc does.

Checked, with flexspin built from this branch (master b5993a8 plus this commit):

• make test_offline: the same tests pass as on master.
• The tests Test/runtests_p2.sh runs, built and compared as it does, on a P2-EDGE: all 29 pass, cexec07 included.
• 1419 C programs, our compiler's test corpus and fuzzer output plus our reproducers: only the reproducer from the issue builds differently from master.