Add subreg_memory_offset helper functions

Add subreg_memory_offset helper functions

This patch adds routines for converting a SUBREG_BYTE offset into a
memory address offset.  The two only differ for paradoxical subregs,
where SUBREG_BYTE is always 0 but the memory address offset can be
negative.

2017-09-04  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* rtl.h (subreg_memory_offset): Declare.
	* emit-rtl.c (subreg_memory_offset): New function.
	* expmed.c (store_bit_field_1): Use it.
	* expr.c (undefined_operand_subword_p): Likewise.
	* simplify-rtx.c (simplify_subreg): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251644