[NFC] Fix usage of Builder.insert(new Bitcast...)in CodeGenFunction

This is exactly a "CreateBitCast", so refactor this to get rid of a
'new'.

Note that this slightly changes the test, as the Builder is now
seemingly smart enough to fold one of the bitcasts into the annotation
call.

Change-Id: I1733fb1fdf91f5c9d88651067130b9a4e7b5ab67
llvm-svn: 349506