When emitting implicit use/def lists, only emit each unique list once. Though

LLVM is able to merge identical static const globals, GCC isn't, and this caused
some bloat in the generated data.  This has a marginal effect on PPC, shrinking
the implicit sets from 10->4, but shrinks X86 from 179 to 23, a much bigger
reduction.

This should speed up the register allocator as well by reducing the dcache
footprint for this static data.

llvm-svn: 22879