The following patch fixes PR40900, which points out an unnecessary
sign-extension for the following code:

extern short shortv2();
short shortv1()
{
  return shortv2();
}

The idea is to represent the sign-extension in the RTL call patterns,
choosing extending variants in calls.c when we notice a suitable
promotion.  Then we let combine do its work; this needed another little
tweak to make sure it recorded the extension of the return register.

Bootstrapped and regression tested on i686-linux.  An earlier version,
which had some unnecessary code and other small differences, was
regression tested on
arm-linux-gnueabi(qemu-system-armv7{arch=armv7-a/thumb,thumb,}).  Ok?


Bernd