Changing the default MaxNumPromotions from 2 to 3.

Summary: In performance tuning, we see performance benefits when enlarge the maximum num promotion targets to 3. This is safe as soon as we have total percentage threshold properly setup (https://reviews.llvm.org/D35962)

Reviewers: davidxl, tejohnson

Reviewed By: tejohnson

Subscribers: llvm-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D35966

llvm-svn: 309346