compiler: eliminate wasteful treg nulling.

* stdlib/optimize.tl (live-info): New slot, clobbered.
(basic-block): New slot, cycle. Struct also inherits
clobbered slot from live-info.
(basic-block print): Print clobbered and cycle.
(basic-blocks local-liveness): Calculate clobbered
for each instruction and from that for the basic block.
(basic-blocks identify-cycle-members): New method.
Discovers which basic blocks are part of any cycle,
and identifies them by setting the new cycle slot
to t.
(basic-blocks do-peephole-block): New local functions
here for determining whether a register has been
clobbered before the first instruction, either
in the same basic block or any ancestors. Only
works when the block is not part of a cycle.
We add a peephole pattern matching move instructions
that set tregs to (t 0)/nil. When we are not in a
cycle block, and the treg has not previously been
clobbered, we know it is clean: it still has the initial
nil value set by the VM and we can remove the instruction.

* stdlib/compiler.tl (compiler optimize): Call the
identify-cycle-members method before peephole.

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