ELF: Attempt to simplify Segment::assignVirtualAddress().

This function is too long and complicated. Looks like new code was
added incrementaly without any refactoring. Maybe no one can describe
its exact semantics any more? It even contains copy-pastes inside it.

This patch is an (incomplete) attempt to simplify the function.
I tried to mechanically translate code to another form more intelligible.
I don't still understand the whole picture, but this patch shouldn't
change the linker's functionality.

llvm-svn: 234944