analyzer: fix ICE on globals with unknown size [PR93388]

analyzer: fix ICE on globals with unknown size [PR93388]

This patch fixes an ICE seen when attempting to build various existing
tests in our testsuite with -fanalyzer, including
gcc.c-torture/compile/980816-1.c.

gcc/analyzer/ChangeLog:
	PR analyzer/93388
	* region-model.cc (region_model::get_initial_value_for_global):
	Fall back to returning an initial_svalue if
	decl_region::get_svalue_for_initializer fails.
	* region.cc (decl_region::get_svalue_for_initializer): Don't
	attempt to create a compound_svalue if the region has an unknown
	size.

gcc/testsuite/ChangeLog:
	PR analyzer/93388
	* gcc.dg/analyzer/data-model-21.c: New test.