Remove word_mode hack for split bitfields

Remove word_mode hack for split bitfields

This patch is effectively reverting a change from 1994.  The reason
I think it's a hack is that store_bit_field_1 is creating a subreg
reference to one word of a field even though it has already proven that
the field spills into the following word.  We then rely on the special
SUBREG handling in store_split_bit_field to ignore the extent of op0 and
look inside the SUBREG_REG regardless.  I don't see any reason why we can't
pass the original op0 to store_split_bit_field instead.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/
	* expmed.c (store_bit_field_1): Do not restrict a multiword op0
	to one word if the field is known to overlap other words.
	(extract_bit_field_1): Likewise.
	(store_split_bit_field): Remove compensating code.
	(extract_split_bit_field): Likewise.

From-SVN: r237211