Ignore PhiNodes when mapping sample profile data

Summary: Like branch instructions, phi nodes frequently do not have debug information related to the block they are in and so they should be ignored.

Reviewers: danielcdh, twoh, Kader, wmi

Reviewed By: wmi

Subscribers: aprantl, llvm-commits

Differential Revision: https://reviews.llvm.org/D55094

llvm-svn: 351102