This operation is critical to staying within the interpretation
loop longer, which avoids the overhead of setup and teardown for
many TBs.

The check in tcg_prologue_init is disabled because TCI does
want to use NULL to indicate exit, as opposed to branching to
a real epilogue.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>