lto: Add toplevel simple assembly heuristics

lto: Add toplevel simple assembly heuristics

This new pass heuristically detects symbols referenced by toplevel
assembly to prevent their optimization.

Heuristics is done by comparing identifiers in assembly to known
symbols.

The pass is split into 2 passes, in LGEN and in WPA.
There must be one pass for WPA to be able to reference any symbol.
However in WPA there may be multiple symbols with the same name,
so we handle those local symbols in LGEN.

gcc/ChangeLog:

	* asm-toplevel.cc (mark_fragile_ref_by_asm):
	Add marked_local to handle symbol as local.
	(ipa_asm_heuristics): New.
	(class pass_ipa_asm): New.
	(make_pass_ipa_asm_lgen): New.
	(make_pass_ipa_asm_wpa): New.
	* common.opt: New flto-toplevel-asm-heuristics.
	* passes.def: New asm passes.
	* timevar.def (TV_IPA_LTO_ASM): New.
	* tree-pass.h (make_pass_ipa_asm_lgen): New.
	(make_pass_ipa_asm_wpa): New.

gcc/testsuite/ChangeLog:

	* gcc.dg/lto/toplevel-simple-asm-1_0.c: New test.
	* gcc.dg/lto/toplevel-simple-asm-1_1.c: New test.
	* gcc.dg/lto/toplevel-simple-asm-2_0.c: New test.
	* gcc.dg/lto/toplevel-simple-asm-2_1.c: New test.