TARGET_STATIC_RTX_ALIGNMENT

This patch adds a new hook that gives the preferred alignment for
a static rtx, so that we don't need to query the front end in
force_const_mem.

2017-10-26  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* target.def (static_rtx_alignment): New hook.
	* targhooks.h (default_static_rtx_alignment): Declare.
	* targhooks.c (default_static_rtx_alignment): New function.
	* doc/tm.texi.in (TARGET_STATIC_RTX_ALIGNMENT): New hook.
	* doc/tm.texi: Regenerate.
	* varasm.c (force_const_mem): Use targetm.static_rtx_alignment
	instead of targetm.constant_alignment.  Remove call to
	set_mem_attributes.
	* config/cris/cris.c (TARGET_STATIC_RTX_ALIGNMENT): Redefine.
	(cris_preferred_mininum_alignment): New function, split out from...
	(cris_constant_alignment): ...here.
	(cris_static_rtx_alignment): New function.
	* config/i386/i386.c (ix86_static_rtx_alignment): New function,
	split out from...
	(ix86_constant_alignment): ...here.
	(TARGET_STATIC_RTX_ALIGNMENT): Redefine.
	* config/mmix/mmix.c (TARGET_STATIC_RTX_ALIGNMENT): Redefine.
	(mmix_static_rtx_alignment): New function.
	* config/spu/spu.c (spu_static_rtx_alignment): New function.
	(TARGET_STATIC_RTX_ALIGNMENT): Redefine.

From-SVN: r254102