Use the first instruction's count to estimate the funciton's entry frequency.

Summary: In the current implementation, we only have accurate profile count for standalone symbols. For inlined functions, we do not have entry count data because it's not available in LBR. In this patch, we use the first instruction's frequency to estimiate the function's entry count, especially for inlined functions. This may be inaccurate due to debug info in optimized code. However, this is a better estimate than the static 80/20 estimation we have in the current implementation.

Reviewers: tejohnson, davidxl

Reviewed By: tejohnson

Subscribers: sanjoy, llvm-commits, aprantl

Differential Revision: https://reviews.llvm.org/D38478

llvm-svn: 315369