Revert "adding xfail for matrix failing instrisic (#949)" (#965)

This reverts commit f69eac6 which added
an xfail due to llvm/llvm-project#185518.
The issue has been fixed, so the test should no longer be xfailed.

Author: Icohedron

test/Basic/Matrix/matrix_elementwise_cast.test

@@ -102,6 +102,3 @@ DescriptorSets:
 # RUN: split-file %s %t
 # RUN: %dxc_target -T cs_6_0 -Fo %t.o %t/source.hlsl
 # RUN: %offloader %t/pipeline.yaml %t.o
-
-# Bug https://github.com/llvm/llvm-project/issues/185518
-# XFAIL: Clang && Vulkan