MSan workaround in Simulator::VisitBitfield.

MSan workaround in Simulator::VisitBitfield.

This change shuffles the code in Simulator::VisitBitfield a bit
to work around a MemorySanitizer bug. New code should not be any
slower or less readable than before.

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

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/212863002

Patch from Evgeniy Stepanov <eugenis@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20401 ce2b1a6d-e550-0410-aec6-3dcde31c8c00