Added comment to the test

Author: DenisBakhvalov

llvm/test/SYCLLowerIR/esimd_lower_crash_zext.ll

@@ -1,6 +1,9 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -LowerESIMD -S < %s | FileCheck %s
 
+; This test checks that there is no crash while lowering 
+; __esimd_unpack_mask.
+
 define spir_func <32 x i16> @_Z3foov() {
 ; CHECK-LABEL: @_Z3foov(
 ; CHECK-NEXT:    ret <32 x i16> zext (<32 x i1> bitcast (<1 x i32> <i32 1717986918> to <32 x i1>) to <32 x i16>)