compiler: streamline live-info object creation.

* stdlib/optimize.tl (basic-blocks local-liveness): When
processing a pure def, we don't copy the live-info
unconditionally, which is waseteful since if the destination
register is a t-reg, we will invoke (new live-info) to
make yet another live info. Instead, let's destructively
mutate the incoming live info from the instruction below,
and return a copy that is made before that is done.
In the def-ref case, the local copy is entirely superfluous,
because in all cases we return a new object.
We also eliminate redundant (set [bb.li-hash insn] li)
evaluations.

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