[TRE] Allow accumulator elimination when base case returns non-constant

[TRE] Allow accumulator elimination when base case returns non-constant

Remove the requirement, that when performing accumulator elimination,
all other cases must return the same dynamic constant. We can do this by
initializing the accumulator with the identity value of the accumulation
operation, and inserting an additional operation before any return.

Differential Revision: https://reviews.llvm.org/D80844