Rework subreg_get_info

Rework subreg_get_info

This isn't intended to change the behaviour, just rewrite the
existing logic in a different (and hopefully clearer) way.
The new form -- particularly the part based on the "block"
concept -- is easier to convert to polynomial sizes.

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

	* rtlanal.c (subreg_get_info): Use more local variables.
	Remark that for HARD_REGNO_NREGS_HAS_PADDING, each scalar unit
	occupies at least one register.  Assume that full hard registers
	have consistent endianness.  Share previously-duplicated if block.
	Rework the main handling so that it operates on independently-
	addressable YMODE-sized blocks.  Use subreg_size_lowpart_offset
	to check lowpart offsets, without trying to find an equivalent
	integer mode first.  Handle WORDS_BIG_ENDIAN != REG_WORDS_BIG_ENDIAN
	as a final register-endianness correction.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r242758