Previously we only do backward copy propagations if there is no read or write to both the Def and Src of the copy between the definition site and the copy site.
However, this restriction can be relaxed. Reads of the Src can be allowed as long as those Reads are rewritten to use Def instead during the propagation.
This commit implements that, potentially improve the generated code. In particular, this resolve an inconsistency between optimized build and optimized build with debuginfo, where DBG_VALUE instructions could prevent backward copy propagation from happening (See: https://bugs.llvm.org/show_bug.cgi?id=49446)