SIGSEGV in vrshift_reg_maskedNode::emit

ASHRInt256VectorTestsMasked test in test/jdk/jdk/incubator/vector/Int256VectorTests.java fails with sigsegv due to null pointer access.

The failure is seen with the following command line on x86_64 ICX both Windows and Linux platform:
-XX:+CreateCoredumpOnCrash -ea -esa -XX:CompileThreshold=100 -XX:+UnlockExperimentalVMOptions -Xcomp -server -XX:-TieredCompilation

The issue is observed at instruct vrshift_reg_masked in x86.ad file at line:

bool is_varshift = !VectorNode::is_vshift_cnt_opcode(in(2)->isa_Mach()->ideal_Opcode());

Here in(2) is a phi node and isa_Mach() returns null.

The phi node handling also needs to be done for is_vshift_cnt() check in the predicates in x86.ad file.