This pass previously excluded rematerialisation candidates if they
clobbered a call-preserved register, on the basis that it then
wouldn't be safe to add new instances of the candidate instruction
after a call.  This patch instead makes the decision on a call-by-call
basis.

The second emit_remat_insns_for_block hunk probably isn't needed,
but it seems safer and more consistent to have it, so that every call
to emit_remat_insns is preceded by a check for invalid clobbers.

2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* early-remat.c: Include regs.h and function-abi.h.
	(early_remat::maybe_add_candidate): Don't check for call-clobbered
	registers here.
	(early_remat::restrict_remat_for_unavail_regs): New function.
	(early_remat::restrict_remat_for_call): Likewise.
	(early_remat::process_block): Before calling emit_remat_insns
	for a previous call in the block, invalidate any candidates
	that would clobber call-preserved registers.
	(early_remat::emit_remat_insns_for_block): Likewise for the
	final call in a block.  Do the same thing for live-in registers
	when calling emit_remat_insns at the head of a block.

From-SVN: r276321