[llvm] [Metadata][profcheck] Handle identical MDNodes in getMergedProfMetadata (PR #175701)

[Snehasish Kumar via llvm-commits]

snehasish wrote:

> @snehasish this breaks the profcheck buildbot, oddly: https://lab.llvm.org/staging/#/builders/221/builds/8017

It was quite odd :) Sent #176098 to fix.

https://github.com/llvm/llvm-project/pull/175701