[SimpleLoopUnswitch] Skip non-trivial unswitching of cold loops

[SimpleLoopUnswitch] Skip non-trivial unswitching of cold loops

With profile data, non-trivial LoopUnswitch will only apply on non-cold loops, as unswitching cold loops may not gain much benefit but significantly increase the code size.

Reviewed By: aeubanks, asbirlea

Differential Revision: https://reviews.llvm.org/D129599