Add an "early rematerialisation" pass

Add an "early rematerialisation" pass

This patch looks for pseudo registers that are live across a call
and for which no call-preserved hard registers exist.  It then
recomputes the pseudos as necessary to ensure that they are no
longer live across a call.  The comment at the head of the file
describes the approach.

A new target hook selects which modes should be treated in this way.
By default none are, in which case the pass is skipped very early.

It might also be worth looking for cases like:

   C1: R1 := f (...)
   ...
   C2: R2 := f (...)
   C3: R1 := C2

and giving the same value number to C1 and C3, effectively treating
it like:

   C1: R1 := f (...)
   ...
   C2: R2 := f (...)
   C3: R1 := f (...)

Another (much more expensive) enhancement would be to apply value
numbering to all pseudo registers (not just rematerialisation
candidates), so that we can handle things like:

  C1: R1 := f (...R2...)
  ...
  C2: R1 := f (...R3...)

where R2 and R3 hold the same value.  But the current pass seems
to catch the vast majority of cases.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* Makefile.in (OBJS): Add early-remat.o.
	* target.def (select_early_remat_modes): New hook.
	* doc/tm.texi.in (TARGET_SELECT_EARLY_REMAT_MODES): New hook.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_select_early_remat_modes): Declare.
	* targhooks.c (default_select_early_remat_modes): New function.
	* timevar.def (TV_EARLY_REMAT): New timevar.
	* passes.def (pass_early_remat): New pass.
	* tree-pass.h (make_pass_early_remat): Declare.
	* early-remat.c: New file.
	* config/aarch64/aarch64.c (aarch64_select_early_remat_modes): New
	function.
	(TARGET_SELECT_EARLY_REMAT_MODES): Define.

gcc/testsuite/
	* gcc.target/aarch64/sve/spill_1.c: Also test that no predicates
	are spilled.
	* gcc.target/aarch64/sve/spill_2.c: New test.
	* gcc.target/aarch64/sve/spill_3.c: Likewise.
	* gcc.target/aarch64/sve/spill_4.c: Likewise.
	* gcc.target/aarch64/sve/spill_5.c: Likewise.
	* gcc.target/aarch64/sve/spill_6.c: Likewise.
	* gcc.target/aarch64/sve/spill_7.c: Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256636 138bc75d-0d04-0410-961f-82ee72b054a4