Rewrite convulated code to avoid adding r0.

Rewrite convulated code to avoid adding r0.

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

	* config/rs6000/rs6000.c (reg_to_non_prefixed): Add forward
	reference.
	(hard_reg_and_mode_to_addr_mask): Delete.
	(rs6000_adjust_vec_address): If the original vector address
	was REG+REG or REG+OFFSET and the element is not zero, do the add
	of the elements in the original address before adding the offset
	for the vector element.  Use address_to_insn_form to validate the
	address using the register being loaded, rather than guessing
	whether the address is a DS-FORM or DQ-FORM address.