Tidy up RelocVisitor.h.

Summary:
RelocVisitor had too many, too small functions. This patch group them
by architecture rather than each relocation type.

Reviewers: grimar, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33580

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303950 91177308-0d34-0410-b5e6-96231b3b80d8