[SimpleLoopUnswitch][NFC] Add option to always drop make.implicit metadata in...

[SimpleLoopUnswitch][NFC] Add option to always drop make.implicit metadata in non-trivial unswitching and save compile time

We might want this if we find out that using of MustExecute analysis is too expensive.
By default we do the analysis because its complexity does not exceed the complexity
of whole loop copying in unswitching. Follow-up for D84925.

Differential Revision: https://reviews.llvm.org/D85001
Reviewed By: asbirlea