analyzer: fix ICE on unhandled tree codes in gassign [PR96640]

analyzer: fix ICE on unhandled tree codes in gassign [PR96640]

PR analyzer/96640 reports a ICE within region_model::on_assignment when
failing to handle a WIDEN_MULT_EVEN_EXPR, and various other tree codes.

The old implementation of region_model::on_assignment gracefully handled
tree codes it didn't understand, returning "UNKNOWN", whereas the new
implementation (r11-2694-g808f4dfeb3a95f50f15e71148e5c1067f90a126d) had
a "sorry_at" and an assertion left over from development, leading to ICEs.

This patch restores the old behavior for these cases, and marks various
vector operations as leading to unknown results.

gcc/analyzer/ChangeLog:
	PR analyzer/96640
	* region-model.cc (region_model::get_gassign_result): Handle various
	VEC_* tree codes by returning UNKNOWN.
	(region_model::on_assignment): Handle unrecognized tree codes by
	setting lhs to an unknown value, rather than issuing a "sorry" and
	asserting.