Revert "Revert "[ConstHoist] Do not rebase single (or few) dependent constant""

This reverts commit bd7b44f35ee9fbe365eb25ce55437ea793b39346.

Reland r342994: disabled the optimization and explicitly enable it in test.

-mllvm -consthoist-min-num-to-rebase<unsigned>=0

[ConstHoist] Do not rebase single (or few) dependent constant

If an instance (InsertionPoint or IP) of Base constant A has only one or few
rebased constants depending on it, do NOT rebase. One extra ADD instruction is
required to materialize each rebased constant, assuming A and the rebased have
the same materialization cost.

Differential Revision: https://reviews.llvm.org/D52243

llvm-svn: 343053