[SimpleLoopUnswitch] Skip non-trivial unswitching of cold functions

In the current main branch, all cold loops will not be applied non-trivial unswitch. As reported in D129599, skipping these cold loops will incur regression in SPEC benchmark.
Thus, instead of skipping cold loops, now only skipping loops in cold functions.

Reviewed By: alexgatea, aeubanks

Differential Revision: https://reviews.llvm.org/D133275