[mips] Inline emitStoreWithSymOffset and emitLoadWithSymOffset methods. NFC

Both methods `MipsTargetStreamer::emitStoreWithSymOffset` and
`MipsTargetStreamer::emitLoadWithSymOffset` are almost the same and
differ argument names only. These methods are used in the single place
so it's better to inline their code and remove original methods.

llvm-svn: 370354