[msan] Avoid redundant origin stores.

[msan] Avoid redundant origin stores.

Origin is meaningless for fully initialized values. Avoid
storing origin for function arguments that are known to
be always initialized (i.e. shadow is a compile-time null
constant).

This is not about correctness, but purely an optimization.
Seems to affect compilation time of blacklisted functions
significantly.

llvm-svn: 213239