Change INSTR_PROF_DEFAULT_NUM_VAL_PER_SITE from 8 to 16.

Summary: In the current implementation, the defaul number of values per site tracked by value profiler is 8, which is too small and could introduce inaccuracies to profile. Changing it to 16 will be able to gain more accurate value profiler.

Reviewers: davidxl, tejohnson

Reviewed By: tejohnson

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D35964

llvm-svn: 309388