compiler: big oreg-related bugfix

There is a smattering of incorrect logic affecting a number of
the compiler's special form sub-compilers. Basically the issue
is that a compiler routine cannot arbitrarily use the oreg
that it is given. If it generates multiple instructions which
clobber a destination, only the last clobber may target oreg.
The reason is that oreg is not necessarily a fresh temporary
that can be used arbitrarily. It can be a variable which is
evaluated by the forms that are compiled by the sub-compiler.
Prematurely storing a value into oreg can affect the behavior
and result value of not-yet executed code. The recent
"indirect function calls" fix addressed just one case of this;
the problem is more wide-spread.

* share/txr/stdlib/compiler.tl
(compiler (maybe-alloc-treg, maybe-free-treg)): New methods.
(compiler (comp-if, comp-switch, comp-unwind-protect,
comp-block, comp-handler-bind, comp-catch, comp-let,
comp-progn, comp-and-or, comp-prog1, comp-for, comp-call,
comp-tree-case): Do not carelessly use oreg for intermediate
computations; allocate one or more temporary registers for
that purpose, and either only move the final value into oreg,
or else indicate a temporary register as the returned frag's
output register.

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