[Function] Clean up {prefix,prologue} data routines (NFC)

Factor out some common code used to get+set function prefix/prologue
data. This may come in handy if we ever decide to store personality
functions in the same way we store prefix/prologue data.

Differential Revision: http://reviews.llvm.org/D13120

Reviewed-by: bogner
llvm-svn: 249460