[ELF] - Refactor of Writer<ELFT>::forEachRelSec.

There is no need to scan over all input sections for relocatable output.
As we do not process or scan relocations anyways.
Patch moves check for Config->Relocatable out to avoid that and also removes
excessive check for isa<EhInputSection> from first for loop. 
It is excessive because we handle all of them in a second for loop below.
That all allowed to simplify code.

Differential revision: https://reviews.llvm.org/D37746

llvm-svn: 313127