Quote from the LLVM Language Reference
If ptr is a stack-allocated object and it points to the first byte of the
object, the object is initially marked as dead. ptr is conservatively
considered as a non-stack-allocated object if the stack coloring algorithm
that is used in the optimization pipeline cannot conclude that ptr is a
stack-allocated object.
By replacing the alloca pointer with the tagged address before this change,
we confused the stack coloring algorithm.
Reviewed By: eugenis
Differential Revision: https://reviews.llvm.org/D121835
078b546555