[BOLT] Do not merge cold and hot chains of basic blocks

There is a post-processing in ext-tsp block reordering that merges some blocks
into chains. This allows to maintain the original block order in the absense of
profile data and can be beneficial for code size (when fallthroughs are merged).
In the earlier version we could merge hot and cold (with zero execution count)
chains, that later were split by SplitFunction.cpp (when split-all-cold=1). The
diff eliminates the redundant merging.

It is unlikely the change will affect the performance of a binary in a
measurable way, as it is mostly operates with cold basic blocks. However, after
the diff the impact of split-all-cold is almost negligible and we can avoid the
extra function splitting.

Measuring on the clang binary (negative is good, positive is a regression):
**clang12**
benchmark1:  `0.0253`
benchmark2:  `-0.1843`
benchmark3:  `0.3234`
benchmark4:  `0.0333`

**clang10**
benchmark1  `-0.2517`
benchmark2  `-0.3703`
benchmark3  `-0.1186`
benchmark4  `-0.3822`

**clang7**
benchmark1  `0.2526`
benchmark2  `0.0500`
benchmark3  `0.3024`
benchmark4  `-0.0489`

**Overall**: `-0.0671 ± 0.1172` (insignificant)

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D129397