[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: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173152 91177308-0d34-0410-b5e6-96231b3b80d8
[msan] Test handling of volatile bitfields.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173154 91177308-0d34-0410-b5e6-96231b3b80d8
[msan] Simplify test code.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173155 91177308-0d34-0410-b5e6-96231b3b80d8