Cap cumulative unroll factor when nesting unrolled constant-array foreach

[ondrejmirtes]

Summary

• tryProcessUnrolledConstantArrayForeach literally evaluates the body once per (key, value) pair. With deep nesting the cumulative product explodes — e.g. 9 nested foreach ([1, 2] as ...) loops causes 2⁹ = 512 inner body evaluations, and a body that appends to an outer array (like $cases[] = $v0) then makes scope merges over a growing constant array compound the cost super-linearly. The per-foreach FOREACH_UNROLL_LIMIT = 16 doesn't catch this because each individual loop only has 2 keys.
• Threads a new foreachUnrollFactor field through StatementContext (getForeachUnrollFactor(), enterUnrolledForeach(int), preserved across enterDeep()). When unrolling would push the cumulative product over FOREACH_UNROLL_NESTED_LIMIT = 16, the foreach falls back to the regular non-unrolled analysis path.
• Adds tests/bench/data/bug-14590.php so the regression is caught by RegressionBench.

Depth-9 reproducer: ~108s → ~3s. PHPStan still cleanly analyses itself; all 12066 existing tests pass.

Closes phpstan/phpstan#14590

Test plan

• make tests — all 12066 tests pass
• make phpstan — no errors
• Reproducer from the issue completes in ~3s