middle-end/110232 - fix native interpret of vector <signed-boolean:1>

The following fixes native interpretation of a buffer as boolean
vector with bit-precision elements such as AVX512 vectors.  The
check whether the buffer covers the whole vector was broken for
bit-precision elements and the following instead implements it
based on the vector type size.

	PR middle-end/110232
	* fold-const.cc (native_interpret_vector): Use TYPE_SIZE_UNIT
	to check whether the buffer covers the whole vector.

	* gcc.target/i386/pr110232.c: New testcase.