Use a bool& parameter for overflow tracking in IRMatcher

Previously the high bit of halide_type_t::lanes was repurposed as a
sticky signed-integer-overflow flag during constant folding. That bit
collides with real lane counts for vectors with many lanes. Replace it
with a bool& overflow parameter threaded through make_folded_const and
constant_fold_bin_op, with each folding entry point declaring its own
local flag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: abadams