Introduce TLI predicative for base-relative Jump Tables.

For 64bit ABIs it is common practice to use relative Jump Tables with
potentially different relocation bases.  As the logic for the jump table
itself doesn't depend on the relocation base, make it easier for targets
to use the generic logic. Start by dropping the now redundant MIPS logic.

Differential Revision: https://reviews.llvm.org/D26578

llvm-svn: 286951