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
true.

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.

llvm-svn: 265498