Add support for individual loop unroll bounds

[jelvani]

These changes allow setting individual loop unroll bounds from the command line. This is useful if we want to check a property for a single loop in a multi-loop function, where a single loop unroll bound may lead to code blow-up.

Usage: If src and tgt have two loops each then individual loop unroll bounds can be used as follows:alive-tv test.ll --src-unroll=1,2 --tgt-unroll=1,2
The unroll bounds are applied to loops in the order they appear in code. These changes preserve backwards compatability; if a single loop unroll bound is provided then it is applied to all loops in the function.
-Alborz

[IamYJLee]

I think it would be useful.
But if we have many functions, such as functions in transform unit tests, we would need too many coupled options.