[SimplifyCFG] Ignore free instructions when computing cost for folding branch to common dest

When determining whether to fold branches to a common destination by
merging two blocks, SimplifyCFG will count the number of instructions to
be moved into the first basic block. However, there's no reason to count
free instructions like bitcasts and other similar instructions.

This resolves missed branch foldings with -fstrict-vtable-pointers in
llvm-test-suite's lambda benchmark.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D108837