[TensorExpr] Fix LLVM 12 build after LLVM API changes (#47480)
Summary:
PolySize was removed: https://reviews.llvm.org/D88982
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47480
Test Plan: Build against LLVM 12.
Reviewed By: glaringlee
Differential Revision: D24773973
Pulled By: asuhan
fbshipit-source-id: d09566675c043d8b63032c52bdadd09e09ccfc39
