[LoopUnswitch] Fix a test case

(h/t to Chandler for pointing this out)

The test in question was not at all testing what it was supposed to
test.  We do not //care// about placing `!make.implicit` in inner
constant branch (since it will be folded away anyway).  We care about
placing `!make.implicit` in the outer branch that switches between
either version of the loop.

Having said that, it is _correct_ to leave behind the `!make.implicit`
in the inner branch, but there is no need to do so.

llvm-svn: 299912