Style cleanup of switches over Slot::Type in the nonoptimizing code

Style cleanup of switches over Slot::Type in the nonoptimizing code
generator.

The Slot::Type enumeration has four values.  It should never be
necessary to use a default to handle the case of a value out of range
of the enumeration.  Doing so silences a useful warning when one of
the enumeration values is actually forgotten or when a new enumeration
value is added.

Review URL: http://codereview.chromium.org/521019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3532 ce2b1a6d-e550-0410-aec6-3dcde31c8c00