Stop trying to merge identical jump tables. This had been inadvertently

disabled for several months (since svn r88806) and no one noticed.  My fix
for pr6543 yesterday reenabled it, but broke the ARM port's code for using
TBB/TBH.  Rather than adding a target hook to disable merging for Thumb2 only,
I'm just taking this out.  It is not common to have identical jump tables,
the code we used to merge them was O(N^2), and it only helps code size, not
performance.

llvm-svn: 98977