tree-vect-generic: Fix bitfield widths [PR94980 3/3]

This third patch of three actually fixes the PR.  We were using
8-bit BIT_FIELD_REFs to access single-bit elements, and multiplying
the vector index by 8 bits rather than 1 bit.

2020-05-12  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/94980
	* tree-vect-generic.c (expand_vector_comparison): Use
	vector_element_bits_tree to get the element size in bits,
	rather than using TYPE_SIZE.
	(expand_vector_condition, vector_element): Likewise.

gcc/testsuite/
	PR tree-optimization/94980
	* gcc.target/i386/pr94980.c: New test.