Simplify SplitInputSection::getRangeAndSize.

This patch adds Size member to SectionPiece so that getRangeAndSize
can just return a SectionPiece instead of a std::pair<SectionPiece *, uint_t>.
Also renamed the function.

llvm-svn: 270346
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 0105991..1f84102 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -855,7 +855,7 @@
     if (!D->Section->Live)
       return false;
     if (auto *S = dyn_cast<MergeInputSection<ELFT>>(D->Section))
-      if (!S->getRangeAndSize(D->Value).first->Live)
+      if (!S->getSectionPiece(D->Value)->Live)
         return false;
   }
   return true;