[msan] More precise instrumentation of select IR.

Some bits of select result may be initialized even if select condition
is not.

https://code.google.com/p/memory-sanitizer/issues/detail?id=50


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204716 91177308-0d34-0410-b5e6-96231b3b80d8