optimization/4847: Optimizer on Sparc breaks routine-calls in vim60

[Nix]

I can confirm that this bug is still present on UltraSPARC in GCC 3.2.1.

I'll work on simplifying soon; I've got to diagnose another vim crash
first, which also disappears when building with GCC-2.95.x and has
different effects on IA32 (hard loop rather than crash), and hence *may*
be another GCC bug --- but that's not by any means certain yet.