[NFC] Remove some old LLVM support from tests.

Our minimum supported LLVM version at the moment is LLVM 19, but we
still had some tests that handled LLVM 15 and LLVM 16. This PR removes
that handling.

Author: hvdijk

llvm/lib/SYCLNativeCPUUtils/compiler_passes/vecz/test/lit/llvm/undef_ub.ll

@@ -14,8 +14,7 @@
 ;
 ; SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-; RUN: %pp-llvm-ver -o %t < %s --llvm-ver %LLVMVER
-; RUN: veczc -k test -w 4 -S < %s | FileCheck %t
+; RUN: veczc -k test -w 4 -S < %s | FileCheck %s
 
 ; ModuleID = 'Unknown buffer'
 source_filename = "Unknown buffer"
@@ -41,6 +40,4 @@ entry:
 ; The "undefs" in the above IR should "optimize" to a trap call and an unreachable
 ; terminator instruction.
 ; CHECK: define spir_kernel void @__vecz_v4_test
-; Before LLVM 17 there's no such trap: the UB is just that the function returns early.
-; CHECK-LT17: ret void
-; CHECK-GE17: unreachable
+; CHECK: unreachable