[ifcombine] avoid creating out-of-bounds BIT_FIELD_REFs [PR118514]

If decode_field_reference finds a load that accesses past the inner
object's size, bail out.

Drop the too-strict assert.


for  gcc/ChangeLog

	PR tree-optimization/118514
	PR tree-optimization/118706
	* gimple-fold.cc (decode_field_reference): Refuse to consider
	merging out-of-bounds BIT_FIELD_REFs.
	(make_bit_field_load): Drop too-strict assert.
	* tree-eh.cc (bit_field_ref_in_bounds_p): Rename to...
	(access_in_bounds_of_type_p): ... this.  Change interface,
	export.
	(tree_could_trap_p): Adjust.
	* tree-eh.h (access_in_bounds_of_type_p): Declare.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118514
	PR tree-optimization/118706
	* gcc.dg/field-merge-25.c: New.