[patch] Fix a couple of VEC_reserve uses, speed up update_ssa a bit

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

Hello,

VEC_reserve allocates an *extra* number of slots. There is
unfortunately no VEC_resize op (one of the first things to add after
the merge of the cxx branch, I suppose...), so to "grow" a VEC without
increasing the used slots count (the VEC_length) it's necessary to
compute the number of extra slots needed and reserve only that number
of slots.

So something like:

      VEC_reserve (ssa_name_info_p, heap, info_for_ssa_name, num_ssa_names);

on an existing VEC with non-null length is wrong. In the worst case,
the VEC_length is already num_ssa_names and the VEC ends up twice as
large as necessary.

Another thing I noticed, is that in update_ssa() we're
sbitmap_zero'ing new_ssa_names and old_ssa_names even after we've
already done so in init_update_ssa. This might seem like a
micro-optimization, but it cuts the time spent in the timevar "tree
SSA incremental" in half for the test case of PR54146...

Bootstrapped&tested on powerpc64-unknown-linux-gnu. OK for trunk?

Ciao!
Steven

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