[ifcombine] out-of-bounds bitfield refs can trap [PR118514]

Check that BIT_FIELD_REFs of DECLs are in range before deciding they
don't trap.

Check that a replacement bitfield load is as trapping as the replaced
load.


for  gcc/ChangeLog

	PR tree-optimization/118514
	* tree-eh.cc (bit_field_ref_in_bounds_p): New.
	(tree_could_trap_p) <BIT_FIELD_REF>: Call it.
	* gimple-fold.cc (make_bit_field_load): Check trapping status
	of replacement load against original load.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118514
	* gcc.dg/field-merge-23.c: New.