Msan failures for new vm/cc/TestRep* tests

[mkustermann]

After cl/150925, from log:

--- Command "run_vm_unittest" (took 575ms):
DART_CONFIGURATION=ReleaseMSANX64 out/ReleaseMSANX64/run_vm_tests --dfe=/b/s/w/ir/cache/builder/sdk/out/ReleaseMSANX64/gen/kernel-service.dart.snapshot --ignore-unrecognized-flags TestRepMovsQwords

exit code:
-6

stdout:
Running test: TestRepMovsQwords

stderr:
==18107==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55f9daed8b96 in void dart::Expect::Equals<unsigned long, unsigned long>(unsigned long const&, unsigned long const&) ../../out/ReleaseMSANX64/../../runtime/platform/assert.h:110:7
    #1 0x55f9db41ef0d in dart::compiler::AssemblerTestRunTestRepMovsQwords(dart::AssemblerTest*) ../../out/ReleaseMSANX64/../../runtime/vm/compiler/assembler/assembler_x64_test.cc:5721:3
    #2 0x55f9db3ee181 in dart::compiler::Dart_TestHelperTestRepMovsQwords(dart::Thread*) ../../out/ReleaseMSANX64/../../runtime/vm/compiler/assembler/assembler_x64_test.cc:5710:1
    #3 0x55f9db3ed30e in dart::compiler::Dart_TestTestRepMovsQwords() ../../out/ReleaseMSANX64/../../runtime/vm/compiler/assembler/assembler_x64_test.cc:5710:1
    #4 0x55f9daeb045e in dart::TestCase::Run() ../../out/ReleaseMSANX64/../../runtime/bin/run_vm_tests.cc:52:3
    #5 0x55f9daeb0873 in dart::TestCaseBase::RunTest() ../../out/ReleaseMSANX64/../../runtime/bin/run_vm_tests.cc:64:11
    #6 0x55f9db2b5141 in dart::TestCaseBase::RunAll() ../../out/ReleaseMSANX64/../../runtime/vm/unit_test.cc:86:13
    #7 0x55f9daeb18c5 in dart::Main(int, char const**) ../../out/ReleaseMSANX64/../../runtime/bin/run_vm_tests.cc:393:3
    #8 0x55f9daeb0f80 in main ../../out/ReleaseMSANX64/../../runtime/bin/run_vm_tests.cc:424:29
    #9 0x7fd20977682f in __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291
    #10 0x55f9dae3ac48 in _start (/b/s/w/ir/cache/builder/sdk/out/ReleaseMSANX64/run_vm_tests+0x3945c48)

SUMMARY: MemorySanitizer: use-of-uninitialized-value ../../out/ReleaseMSANX64/../../runtime/platform/assert.h:110:7 in void dart::Expect::Equals<unsigned long, unsigned long>(unsigned long const&, unsigned long const&)
Exiting

--- Re-run this test:
python tools/test.py -n dartk-msan-linux-release-x64 vm/cc/TestRepMovsQwords