[llvm] [DAG] Update canCreateUndefOrPoison to handle ISD::VECTOR_COMPRESS (PR #168010)

[Simon Pilgrim via llvm-commits]

================
@@ -0,0 +1,20 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -mattr=+avx512f,+avx512vl | FileCheck %s
+
+declare <16 x i32> @llvm.experimental.vector.compress.v16i32(<16 x i32>, <16 x i1>, <16 x i32>)
+
+define <16 x i32> @test_compress_freeze_elimination(<16 x i32> %a0, <16 x i32> %a1, <16 x i8> %a3) {
----------------
RKSimon wrote:

Add negative tests for undef passthrough?

https://github.com/llvm/llvm-project/pull/168010