LCM: Export 2 helpful functions as global for VSETVL PASS use in RISC-V backend

This patch exports 'compute_antinout_edge' and 'compute_earliest' as global scope
which is going to be used in VSETVL PASS of RISC-V backend.

The demand fusion is the fusion of VSETVL information to emit VSETVL which dominate and pre-config for most
of the RVV instructions in order to elide redundant VSETVLs.

For exmaple:

for
 for
  for
    if (cond}
      VSETVL demand 1: SEW/LMUL = 16 and TU policy
    else
      VSETVL demand 2: SEW = 32

VSETVL pass should be able to fuse demand 1 and demand 2 into new demand: SEW = 32, LMUL = M2, TU policy.
Then emit such VSETVL at the outmost of the for loop to get the most optimal codegen and run-time execution.

Currenty the VSETVL PASS Phase 3 (demand fusion) is really messy and un-reliable as well as un-maintainable.
And, I recently read dragon book and morgan's book again, I found there "earliest" can allow us to do the
demand fusion in a very reliable and optimal way.

So, this patch exports these 2 functions which are very helpful for VSETVL pass.

gcc/ChangeLog:

	* lcm.cc (compute_antinout_edge): Export as global use.
	(compute_earliest): Ditto.
	(compute_rev_insert_delete): Ditto.
	* lcm.h (compute_antinout_edge): Ditto.
	(compute_earliest): Ditto.

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