[GVNHoist] add cast to unbreak windows build

Follow up to
commit 831e99fe ("[GVNHoist] don't hoist callbr users into the callbr's block")

Looks like MSVC has trouble with llvm::is_contained. Unbreak the build.

Link: https://lab.llvm.org/buildbot/#/builders/127/builds/45021/steps/7/logs/stdio

Reviewed By: hans, kuhar

Differential Revision: https://reviews.llvm.org/D145969