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[MSAN] Avoid dangling ActualFnStart when replacing instruction

Authored by guiand on Aug 14 2020, 4:34 PM.

Description

[MSAN] Avoid dangling ActualFnStart when replacing instruction

This would be a problem if the entire instrumented function was a call
to
e.g. memcpy

Use FnPrologueEnd Instruction* instead of ActualFnStart BB*

Differential Revision: https://reviews.llvm.org/D86001