Minor codegen silliness in ExecInterpExpr()

I noticed the following while poking around with perf:

| fcinfo->isnull = false;
|b5b: movb $0x0,0x1c(%rdx)
| *op->resvalue = op->d.func.fn_addr(fcinfo);
0.02 | mov 0x8(%rbx),%rcx
1.19 | mov %rdx,%rdi
0.93 | mov %rdx,(%rsp)
| mov %rcx,0x8(%rsp)
0.01 | callq *0x28(%rbx)
2.17 | mov 0x8(%rsp),%rcx
| mov %rax,(%rcx)
| *op->resnull = fcinfo->isnull;
1.18 | mov (%rsp),%rdx
4.32 | mov 0x10(%rbx),%rax
0.06 | movzbl 0x1c(%rdx),%edx
9.14 | mov %dl,(%rax)

It looks to me like gcc believes it is required to evaluate "op->resvalue"
before invoking the called function, just in case the function somehow has
access to *op and modifies that. We could save a pointless register spill
and reload if there were a temporary variable in there, ie

EEO_CASE(EEOP_FUNCEXPR)
{
FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
+ Datum fvalue;

fcinfo->isnull = false;
- *op->resvalue = op->d.func.fn_addr(fcinfo);
+ fvalue = op->d.func.fn_addr(fcinfo);
+ *op->resvalue = fvalue;
*op->resnull = fcinfo->isnull;

EEO_NEXT();
}

and likewise in the other FUNCEXPR cases.

This is on a rather old gcc, haven't checked on bleeding-edge versions.

regards, tom lane