make TestRegisterVariables slightly more resilient
This test sets the compiler optimization level to -O1 and
makes some assumptions about how local frame vars will be
stored (i.e. in registers). These assumptions are not always
I did a first-pass set of improvements that:
(1) no longer assumes that every one of the target locations has
every variable in a register. Sometimes the compiler is even smarter and skips the register entirely.
(2) simply expects one of the 5 or so variables it checks
to be in a register.
This test probably passes on a whole lot more systems than it
used to now. This is certainly true on OS X.