Restore an assignment deleted in commit 789f79b.

Restore an assignment deleted in commit 789f79b.

Because of the accidental deletion, mcgg on my machine followed a
garbage pointer, and never wrote calls to emit_fragment.

A wrong call to `data->emit_reg(0, 0)` instead of the correct
`data->emit_fragment(0)` caused PowerPC mcg to emit an empty string
instead of `8(fp)`, causing a syntax error in PowerPC as.

The wrong `data->emit_reg(0, 0)` called the function emit_reg() in
mach/proto/mcg/pass_instructionselection.c, but that function
unfortunately has `if (vreg) { ... }`.  The call had vreg == NULL
because the fragment wasn't a vreg, but emit_reg() ignored the problem
and emit nothing.