Add a peephole optimization for PUSHFIELDS(1).

The PUSHFIELDS opcode is a costly one, yet lots of constructors have at
most one usable argument (e.g., option, nat, positive, Z, Acc). For those
constructors, PUSHFIELDS(1) is replaced by GETFIELD(0);PUSH, assuming
the accu register is no longer used afterwards.

Replacing one single opcode by two opcodes might seem like a
pessimization, but it is not. Indeed, pattern-matching branches usually
start by filling the accu register with a constructor argument or the
value of a free variable or a constant. All of those offer peephole
optimizations for PUSH, which means that the number of opcodes actually
stay constant.

Note that, for the same reason, the assumption above holds in practice:
the accu register is no longer used after PUSHFIELDS.

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