regcomp.c: Use name consistently

regcomp.c: Use name consistently

In the function Perl_re_op_compile(), there is a stack variable 'ri',
and an element of a stack structure rxi, which part way through one is
set to the other, so that at times one is used and other times the other
is used.  This commit removes the variable, in favor of the struct
element, making consistent use throughout the function (and since the
struct is passed to its callees, it makes this value available to them
always)