Restore behavior of PrepareForBreakpoints which was broken by r21145.

Rename Debug::MaybeRecompileFunctionForDebugging to
EnsureFunctionHasDebugBreakSlots and ensure that it does
nothing if the function is unoptimized code with debug
break slots, otherwise, if the shared code has no
debug break slots, it recompile that shared code and
sets the function code to that shared code.

Also removes two incorrect ASSERTs.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/271873003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21201 ce2b1a6d-e550-0410-aec6-3dcde31c8c00