[JumpThreading] Conditionally freeze its condition when unfolding select

This patch fixes pr45956 (https://bugs.llvm.org/show_bug.cgi?id=45956 ).
To minimize its impact to the quality of generated code, I suggest enabling
this only for LTO as a start (it has two JumpThreading passes registered).
This patch contains a flag that makes JumpThreading enable it.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D84940