Optimizations to smearing interface

[agrebe]

I have been computing two-link smearing with QUDA and have made two optimizations to the smearing interface:

1. In lib/milc_interface.cpp, skip the call to loadGaugeQuda if the useResidentGauge flag is set (since in this case, the 2-link field is already computed)
2. Simplify the four linear algebra operations in the loop in performTwoLinkGaussianSmearNStep in lib/interface_quda.cpp to a single operation
   I have tested that this agrees with the previous implementation (using QUDA driven by MILC on a single GPU), and overall these changes increase performance by a factor of about 2 in our project.

[maddyscientist]

This is great contribution, thanks @agrebe. Aside from my comment, please also clang-format the diff, and also feel free to add your name to the contributors in the README.

@Jenkins ok to test

[maddyscientist]

Thanks for the fast turn around. Approving. We can merge this once CI finishes.

(And I see you were already contributor, apologies for failing to recall this.)

[agrebe]

Thank you for reviewing this so quickly -- I'm glad to be able to contribute given how useful this codebase has been for me!