[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
diff --git a/torch/csrc/jit/tensorexpr/llvm_codegen.cpp b/torch/csrc/jit/tensorexpr/llvm_codegen.cpp
index bafb559..a2a6d15 100644
--- a/torch/csrc/jit/tensorexpr/llvm_codegen.cpp
+++ b/torch/csrc/jit/tensorexpr/llvm_codegen.cpp
@@ -87,7 +87,7 @@
 #if LLVM_VERSION_MAJOR <= 11
   return llvm::ElementCount(static_cast<unsigned>(lanes), false);
 #elif LLVM_VERSION_MAJOR == 12
-  return llvm::ElementCount(llvm::PolySize<unsigned>::getFixed(lanes));
+  return llvm::ElementCount::getFixed(lanes);
 #else
 #error Only LLVM versions 8 through 12 are supported.
 #endif