[ConstHoisting] Add BFI in constanthoisting pass and select the best insertion

places based on it.

Existing constant hoisting pass will merge a group of contants in a small range
and hoist the const materialization code to the common dominator of their uses.
However, if the uses are all in cold pathes, existing implementation may hoist
the materialization code from cold pathes to a hot place. This may hurt performance.
The patch introduces BFI to the pass and selects the best insertion places based
on it.

The change is controlled by an option consthoist-with-block-frequency which is
off by default for now.

Differential Revision: https://reviews.llvm.org/D28962

llvm-svn: 300989