Avoid constructing a Regexp when splitting a String with negative limit
Avoid constructing a Regexp when splitting a String with negative limit

When splitting a string using a String pattern, we don't want to turn it
into a Regexp unless we absolutely have to. Prior to this change,
passing a negative limit into String#split would have forced this
conversion and incurred a high performance cost as a result.

Running the benchmark for this particular case shows a three-fold
improvement.

Wirachmat/rubinius,ngpestelos/rubinius,ruipserra/rubinius,dblock/rubinius,digitalextremist/rubinius,heftig/rubinius,benlovell/rubinius,Azizou/rubinius,Azizou/rubinius,ruipserra/rubinius,jemc/rubinius,dblock/rubinius,pH14/rubinius,Azizou/rubinius,ruipserra/rubinius,digitalextremist/rubinius,kachick/rubinius,ruipserra/rubinius,jsyeo/rubinius,sferik/rubinius,benlovell/rubinius,jemc/rubinius,kachick/rubinius,digitalextremist/rubinius,sferik/rubinius,Wirachmat/rubinius,mlarraz/rubinius,heftig/rubinius,ruipserra/rubinius,mlarraz/rubinius,kachick/rubinius,Azizou/rubinius,mlarraz/rubinius,mlarraz/rubinius,ngpestelos/rubinius,digitalextremist/rubinius,pH14/rubinius,sferik/rubinius,dblock/rubinius,jemc/rubinius,jsyeo/rubinius,benlovell/rubinius,jsyeo/rubinius,digitalextremist/rubinius,benlovell/rubinius,sferik/rubinius,kachick/rubinius,pH14/rubinius,mlarraz/rubinius,Wirachmat/rubinius,digitalextremist/rubinius,ngpestelos/rubinius,jemc/rubinius,benlovell/rubinius,kachick/rubinius,ruipserra/rubinius,Wirachmat/rubinius,jsyeo/rubinius,dblock/rubinius,jemc/rubinius,jsyeo/rubinius,heftig/rubinius,mlarraz/rubinius,Wirachmat/rubinius,digitalextremist/rubinius,Azizou/rubinius,dblock/rubinius,heftig/rubinius,heftig/rubinius,ngpestelos/rubinius,kachick/rubinius,jsyeo/rubinius,Wirachmat/rubinius,benlovell/rubinius,Wirachmat/rubinius,jsyeo/rubinius,sferik/rubinius,mlarraz/rubinius,ngpestelos/rubinius,sferik/rubinius,jemc/rubinius,kachick/rubinius,ngpestelos/rubinius,kachick/rubinius,Azizou/rubinius,ngpestelos/rubinius,pH14/rubinius,benlovell/rubinius,pH14/rubinius,jemc/rubinius,heftig/rubinius,pH14/rubinius,ruipserra/rubinius,sferik/rubinius,pH14/rubinius,Azizou/rubinius,dblock/rubinius,dblock/rubinius,heftig/rubinius