[Mips] Do not read addends for relocations which do not use them

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221602 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Replace the redundant condition by assert call

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221603 91177308-0d34-0410-b5e6-96231b3b80d8