[CodeGenPrepare] Fix ModifiedDT flag in optimizeSelectInst

r44412 fixed a huge compile time regression but it needed ModifiedDT flag to be
maintained correctly in optimizations in optimizeBlock() and optimizeInst().
Function optimizeSelectInst() does not update the flag.
This patch propagates the flag in optimizeSelectInst() back to
optimizeBlock().

This patch also removes ModifiedDT in CodeGenPrepare class (which is not used).
The property of ModifiedDT is now recorded in a ref parameter.

Differential Revision: https://reviews.llvm.org/D59139

llvm-svn: 355751