tree-optimization/117424 - invalid LIM of trapping ref

tree-optimization/117424 - invalid LIM of trapping ref

The following addresses a bug in tree_could_trap_p leading to
hoisting of a possibly trapping, because of out-of-bound, access.
We only ensured the first accessed byte is within a decl there,
the patch makes sure the whole base of the reference is within it.
This is pessimistic if a handled component would then subset to
a sub-object within the decl but upcasting of a decl to larger
types should be uncommon, questionable, and wrong without
-fno-strict-aliasing.

The testcase is a bit fragile, but I could not devise a (portable)
way to ensure an out-of-bound access to a decl would fault.

	PR tree-optimization/117424
	* tree-eh.cc (tree_could_trap_p): Verify the base is
	fully contained within a decl.

	* gcc.dg/tree-ssa/ssa-lim-25.c: New testcase.