[PATCH v2] In the pipeline, USE or CLOBBER should delay execution if it starts a...

[PATCH v2] In the pipeline, USE or CLOBBER should delay execution if it starts a new live range.

CLOBBER and USE does not represent real instructions, but in the
process of pipeline optimization, they will wait for transmission
in ready list like other insns, without considering resource
conflicts and cycles. This results in a multi-issue CPU architecture
that can be issued at any time if other regular insns have resource
conflicts or cannot be launched for other reasons. As a result,
its position is advanced in the generated insns sequence, which
will affect register allocation and often lead to more redundant
mov instructions.

A simple example:
https://github.com/majin2020/gcc-test/blob/master/test.c
This is a function in the dhrystone benchmark.

https://github.com/majin2020/gcc-test/blob/0b08c1a13de9663d7d9aba7539b960ec0607ca24/test.c.299r.sched1
This is a log of the pass 'sched1' When -mtune=rocket but issue_rate == 2.

The pipeline is:
;; | insn | prio |
;; |  17  |  3   | r142=a0 alu
;; |  14  |  0   | clobber r136 nothing
;; |  13  |  0   | clobber a0 nothing
;; |  18  |  2   | r143=a1 alu
...
;; |  12  |  0   | a0=r136 alu
;; |  15  |  0   | use a0 nothing

In this log, insn 13 and 14 are much ahead of schedule, which risks generating
redundant mov instructions, which seems unreasonable.

Therefore, I submit patch again on the basis of the last review
opinions to try to solve this problem.

https://github.com/majin2020/gcc-test/commit/efcb43e3369e771bde702955048bfe3f501263dd#diff-805031b1be5092a2322852a248d0b0f92eef7cad5784a8209f4dfc6221407457L189
This is the diff log of shed1 after patch is added.

The new pipeline is:
;; | insn | prio |
;; |  17  |  3   | r142=a0 alu
...
;; |  10  |  0   | [r144]=r141 alu
;; |  13  |  0   | clobber a0 nothing
;; |  14  |  0   | clobber r136 nothing
;; |  12  |  0   | a0=r136 alu
;; |  15  |  0   | use a0 nothing

gcc/ChangeLog:
	* haifa-sched.cc (use_or_clobber_starts_range_p): New.
	(prune_ready_list): USE or CLOBBER should delay execution
	if it starts a new live range.