[runtime] JIT debugging option to optimize only single methods.

The --single-method option run regression tests from an assembly, like
--regression.  Instead of going through a list of optimizations,
however, it takes one optimization combination, runs all tests with
all optimizations turned off, while gathering a list of which methods
were jitted.  Then, for each jitted method, it runs all tests again,
with the optimizations applied to only that one method, stopping as
soon as a test fails.  Example:

    mono --single-method=branch,peephole basic-float.exe