[msan] Stop using volatile assignments as undef checks in MSan tests.
[msan] Stop using volatile assignments as undef checks in MSan tests.

A runtime call is used instead.
MSan will stop adding shadow checks on volatile stores soon.


git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@173152 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt