[mips][ias] Make the default path unreachable in needsRelocateWithSymbol() (except for N64).

Following post-commit comments on r268900 from Rafael Espindola:
The missing relocations are now explicitly listed in the switch statement with
appropriate FIXME comments and the default path is now unreachable. The
temporary exception to this is that compound relocations for N64 still have a
default path that returns true. This is because fixing that case ought to be a
separate patch.

Also make R_MIPS_NONE return false since it has no effect on the section data.

llvm-svn: 269047