[LTT] Don't attempt to lower type tests used only by assumes

Type tests used only by assumes were original for devirtualization, but
are meant to be kept through the first invocation of LTT so that they
can be used for additional optimization. In the regular LTO case where
the IR is analyzed we may find a resolution for the type test and end up
rewriting the associated vtable global, which can have implications on
section splitting. Simply ignore these type tests.

Fixes PR48245.

Differential Revision: https://reviews.llvm.org/D96083