fixups for calls to gen_rtx()

[Bernd Schmidt]

> 
> This, in particular the case in combine.c, can cause havoc on 64-bit
> architectures, depending upon where the "pass in register" cutoff is
> for arguments.

[patch changing 0 to NULL_RTX in calls to gen_rtx]

I once had a patch that scans rtl.def and generates a lot of functions
with names like "gen_rtx_REG" which don't use varargs and have a proper
prototype. Is someone interested in this?

Bernd