Fix assertion failure with $float = $regexp assignment

Fix assertion failure with $float = $regexp assignment

Commit b9ad13acb3 caused case SVt_REGEXP in sv_upgrade to fall
through to the assertions under case SVt_PVIV that are not relevant to
SVt_REGEXP.

We should really be setting the FAKE flag when actually making a sca-
lar a regexp, rather than in sv_upgrade.  (I will probably need it
there in future commits, too, since it really should be possible for
SVt_PVLVs to hold regular expressions.)