C2: MergeStores: handle negative shift values

Test: applications/javafuzzer/BigTest.java

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/System/Volumes/Data/mesos/work_dir/slaves/412fde1c-3e26-4c3d-b41e-bab008e2c85d-S9637/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/fcd59e7d-0eaa-4e79-a3dd-1eba2e7376f0/runs/30ba630e-86cc-4107-a518-430caed87a5e/workspace/open/src/hotspot/share/opto/memnode.cpp:3111), pid=38651, tid=27907
# assert(shift_out >= 0) failed: must be positive
#
--------------- T H R E A D ---------------

Current thread (0x000000013c821e10): JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=27907, stack(0x000000016ebc0000,0x000000016edc3000) (2060K)]


Current CompileTask:
C2:74 8 !b 4 Test::mainTest (579 bytes)

Stack: [0x000000016ebc0000,0x000000016edc3000], sp=0x000000016edbef80, free space=2043k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.dylib+0x11216a0] VMError::report_and_die(int, char const*, char const*, char*, Thread*, unsigned char*, void*, void*, char const*, int, unsigned long)+0x564 (memnode.cpp:3111)
V [libjvm.dylib+0x1121ec0] VMError::report_and_die(Thread*, unsigned int, unsigned char*, void*, void*)+0x0
V [libjvm.dylib+0x549c9c] print_error_for_unit_test(char const*, char const*, char*)+0x0
V [libjvm.dylib+0xd306ac] MergePrimitiveArrayStores::is_con_RShift(Node const*, Node const*&, int&)+0x1e8
V [libjvm.dylib+0xd2fcfc] MergePrimitiveArrayStores::is_adjacent_input_pair(Node const*, Node const*, int) const+0x64
V [libjvm.dylib+0xd2fae0] MergePrimitiveArrayStores::is_adjacent_pair(StoreNode const*, StoreNode const*) const+0x80
V [libjvm.dylib+0xd2e988] MergePrimitiveArrayStores::run()+0x114
V [libjvm.dylib+0xd311dc] StoreNode::Ideal(PhaseGVN*, bool)+0x504
V [libjvm.dylib+0xe695bc] PhaseIterGVN::transform_old(Node*)+0x164
V [libjvm.dylib+0xe68a98] PhaseIterGVN::optimize()+0xf8
V [libjvm.dylib+0x4b653c] Compile::process_for_post_loop_opts_igvn(PhaseIterGVN&)+0x138
V [libjvm.dylib+0x4b198c] Compile::Optimize()+0x9b8
V [libjvm.dylib+0x4afdd0] Compile::Compile(ciEnv*, ciMethod*, int, Options, DirectiveSet*)+0x1238
V [libjvm.dylib+0x379b7c] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x1e0
V [libjvm.dylib+0x4ce1ec] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x868
V [libjvm.dylib+0x4cd61c] CompileBroker::compiler_thread_loop()+0x348
V [libjvm.dylib+0x88eb5c] JavaThread::thread_main_inner()+0x1dc
V [libjvm.dylib+0x1062c2c] Thread::call_run()+0xf4
V [libjvm.dylib+0xe1039c] thread_native_entry(Thread*)+0x138
C [libsystem_pthread.dylib+0x6f94] _pthread_start+0x88
Lock stack of current Java thread (top to bottom):