tree-optimization/106934 - avoid BIT_FIELD_REF of bitfields

The following avoids creating BIT_FIELD_REF of bitfields in
update-address-taken.  The patch doesn't implement punning to
a full precision integer type but leaves a comment according to
that.

	PR tree-optimization/106934
	* tree-ssa.cc (non_rewritable_mem_ref_base): Avoid BIT_FIELD_REFs
	of bitfields.
	(maybe_rewrite_mem_ref_base): Likewise.

	* gfortran.dg/pr106934.f90: New testcase.