[TensorExpr] add simplication of constant branches to IR Simplifier (#36257)
Summary:
Adds handling of constant branches to the TensorExpr IR Simplifier. This covers both IfThenElse and Cond when the condition expression is a known constant (e.g. `IfThenElse(1, X, Y) => X`), or when both arms of the branch are the same (e.g. `IfThenElse(Y, X, X) => X`).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36257
Differential Revision: D20947777
Pulled By: nickgg
fbshipit-source-id: 974379e42a6d65ce3e7178622afb62d36ad4e380
