[MemProf] Ensure node merging happens for newly created nodes (#151593)

We weren't performing node merging on newly created nodes in some cases.
Use a simple iteration over the node and its callers until no more
opportunities are found. I confirmed that for several large codes the
max iterations is 3 (meaning we only needed to do any work on the first
2, as expected). This can potentially be made more elegant in the
future, but it is a simple and effective solution.

Also fix a bug exposed by the test case, getting the function for a call
instruction in the FullLTO handling, using an existing method to look
through aliases if needed.

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