regcomp.c: Refactor some if-elses

regcomp.c: Refactor some if-elses

if (foo) {
    ...
    return
} else {
    stuff
}

is equivalent to

if (foo) {
    ...
    return
}

stuff

This commit changes the former to the latter, which also means moving
some declarations earlier, and some intialization later, and removing
some no longer valid code.  This will be useful in the next commit.