[ELF] - ICF: add one more test case #3.

ICF is able to merge sections which relocations referring regular input sections
or mergeable sections, so it handles InputSection and MergeInputSection cases.

The following "return false" line which is executed in case of another type
of the sections is uncovered by our test cases:
https://github.com/llvm-mirror/lld/blob/master/ELF/ICF.cpp#L285
Patch fixes code coverage for this place.

llvm-svn: 335482