Enable some rewrite rules's use for branch merging in FGL.
Adds the given rewrite rule runes to the fgl-branch-merge-rules table so
that they can be used to merge if branches in FGL. See add-fgl-rewrites for details on rewrite rule runes recognized in FGL. Each rule
must have as its LHS a call of if with a function call as its then
- Alias for add-fgl-branch-merges that just takes one name.