MemorySanitizer doesn't automatically work with dynamically generated
code or inline assembly, since it can't instrument it to know what
memory it touches. We can help it by marking all the memory that is
written to by Reactor with the Subzero back-end as initialized by
calling __msan_unpoison.

Note that writes to memory don't guarantee proper initialization. It
could be copying or writing other uninitialized values.

See also https://sites.google.com/a/chromium.org/dev/developers/testing/memorysanitizer

Bug chromium:860533

Change-Id: Idf64e43c6ab9b8f71f64723fc7e3653f6ea2fb30
Reviewed-on: https://swiftshader-review.googlesource.com/19789Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>