Cleanup LTO tag streaming.

Cleanup LTO tag streaming.

this patch cleanups a bit the way LTO tags are assigned (so we do not use magic
numbers of 2) and reorders them approximately by their frequency so the stream
gets little bit smaller because of uleb coding.

From:

[WPA] Compression: 111213968 input bytes, 293801489 uncompressed bytes (ratio: 2.641768)
[WPA] Size of mmap'd section decls: 85983129 bytes
[WPA] Size of mmap'd section function_body: 14058627 bytes

to:

[WPA] Compression: 110593119 input bytes, 287696614 uncompressed bytes (ratio: 2.601397)
[WPA] Size of mmap'd section decls: 85628556 bytes
[WPA] Size of mmap'd section function_body: 13842928 bytes

lto-bootstrapped/regtested x86_64-linux, will commit it shortly.

	* lto-streamer.h (LTO_tags): Reorder so frequent tags has small indexes;
	add LTO_first_tree_tag and LTO_first_gimple_tag.
	(lto_tag_is_tree_code_p): Update.
	(lto_tag_is_gimple_code_p): Update.
	(lto_gimple_code_to_tag): Update.
	(lto_tag_to_gimple_code): Update.
	(lto_tree_code_to_tag): Update.
	(lto_tag_to_tree_code): Update.