regmatch(): remove reginput from IFMATCH etc

regmatch(): remove reginput from IFMATCH etc

It was being used essentially as a temporary var within the branch,
so replace it with a temp var in a new block scope.

On return in IFMATCH_A / IFMATCH_A_fail, there's no need to set reginput
any more, so don't. The SUSPEND case used to set locinput = reginput, but
at that point, the two variables already always had the same value anyway.

This is part of a campaign to eliminate the reginput variable.