poly_int: ira subreg liveness tracking

poly_int: ira subreg liveness tracking

Normmaly the IRA-reload interface tries to track the liveness of
individual bytes of an allocno if the allocno is sometimes written
to as a SUBREG.  This isn't possible for variable-sized allocnos,
but it doesn't matter because targets with variable-sized registers
should use LRA instead.

This patch adds a get_subreg_tracking_sizes function for deciding
whether it is possible to model a partial read or write.  Later
patches make it return false if anything is variable.

2017-12-20  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* ira.c (get_subreg_tracking_sizes): New function.
	(init_live_subregs): Take an integer size rather than a register.
	(build_insn_chain): Use get_subreg_tracking_sizes.  Update calls
	to init_live_subregs.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r255881