[patch] Reduce memory overhead for build_insn_chain

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Hello,

In the test case for PR54146, build_insn_chain spends almost all its
time in this loop:

  FOR_EACH_BB_REVERSE (bb)
    {
      bitmap_iterator bi;
      rtx insn;

      CLEAR_REG_SET (live_relevant_regs);
-->   memset (live_subregs_used, 0, max_regno * sizeof (int));

The test case has >400,000 basic blocks and >1,000,000 regs...

Fortunately this can be done much more efficiently if one realizes
that an sbitmap knows its own size (using SBITMAP_SIZE that I
introduced recently). With that, live_subregs_used can be a sparse
bitmap.

Bootstrapped&tested on x86_64-unknown-linux-gnu. OK for trunk?

Ciao!
Steven

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