[JumpThreading] Put a limit on the PHI nodes when duplicating a BB.

Do not duplicate a BB if it has a lot of PHI nodes.
If a threadable chain is too long then the number of duplicated PHI nodes
can add up, leading to a substantial increase in compile time when rewriting
the SSA.

Fixes https://github.com/llvm/llvm-project/issues/58203
Differential Revision: https://reviews.llvm.org/D136716

The threshold of 76 in this patch is reasonably high and reduces the compile
time of cldwat2m_macro.f90 in SPEC2017/cam4 from 80+min to <2min.

Change-Id: I153c89a8e0d89b206a5193dc1b908c67e320717e