Provide versions of subreg_lowpart_offset and subreg_highpart_offset
that work on mode sizes rather than modes.  Also provide a routine
that converts an lsb position to a subreg offset.

The intent (in combination with later patches) is to move the
handling of the BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN case into
just two places, so that for other combinations we don't have
to split offsets into words and subwords.

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* rtl.h (subreg_size_offset_from_lsb): Declare.
	(subreg_offset_from_lsb): New function.
	(subreg_size_lowpart_offset): Declare.
	(subreg_lowpart_offset): Turn into an inline function.
	(subreg_size_highpart_offset): Declare.
	(subreg_highpart_offset): Turn into an inline function.
	* emit-rtl.c (subreg_size_lowpart_offset): New function.
	(subreg_size_highpart_offset): Likewise
	* rtlanal.c (subreg_size_offset_from_lsb): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r242755