[SimplifyCFG] Ignore ephemeral values when counting insts for threading

Ignore ephemeral values (only feeding llvm.assume intrinsics) when
computing the instruction count to decide if a block is small enough for
threading. This is similar to the handling of these values in the
InlineCost computation. These instructions will eventually be removed
and shouldn't count against code size (similar to the existing ignoring
of phis).

Without this change, when enabling -fwhole-program-vtables, which causes
type test / assume sequences to be inserted by clang, we can get
different threading decisions. In particular, when building with
instrumentation FDO it can affect the optimizations decisions before FDO
matching, leading to some mismatches.

Differential Revision: https://reviews.llvm.org/D101494