analyzer: fix ICE on unhandled tree codes in get_rvalue_1 [PR96641]

analyzer: fix ICE on unhandled tree codes in get_rvalue_1 [PR96641]

The old implementation of region_model::get_rvalue_1 gracefully handled
tree codes it didn't understand, returning "UNKNOWN", whereas the new
implementation (r11-2694-g808f4dfeb3a95f50f15e71148e5c1067f90a126d) had
an assertion left over from development, leading to ICEs.

This patch restores the old behavior for these cases.

gcc/analyzer/ChangeLog:
	PR analyzer/96641
	* region-model.cc (region_model::get_rvalue_1): Handle
	unrecognized tree codes by returning "UNKNOWN.

gcc/testsuite/ChangeLog:
	PR analyzer/96641
	* g++.dg/analyzer/pr96641.C: New test.