Adjust how variable vector extraction is done.

Adjust how variable vector extraction is done.

2020-02-03  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000.c (get_vector_offset): New helper function
	to calculate the offset in memory from the start of a vector of a
	particular element.  Add code to keep the element number in
	bounds if the element number is variable.
	(rs6000_adjust_vec_address): Move calculation of offset of the
	vector element to get_vector_offset.
	(rs6000_split_vec_extract_var): Do not do the initial AND of
	element here, move the code to get_vector_offset.