compiler: avoid invalid if d-reg optimization.

We cannot assume that a d register is has a non-nil value.
This is because d registers are exploited in the
implementation of load-time: the result of a load-time form is
stored by mutating a d register, and the value could be nil.

Since we still want to be able to assume that d registers
are non-nil, what we can do is just avoid that assumption for
those d regisers that are used for load-time values.

* share/txr/stdlib/compiler.tl (struct compiler): When
constructing basic-blocks, pass a new constructor argument:
the list of load-time d-regs. This is easily obtained by
mapping the load-time frags to their oreg slots, which are
those d-regs.

* share/txr/stdlib/optimize.tl (struct basic-blocks): New
slot and BOA constructor argument, lt-dregs.
(basic-blocks thread-jumps-block): Add a require to the
pattern (if (d @reg) @jlabel), that the register must not
be one of the load-time d-regs.

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