Revert "Revert "[MemCpyOpt] implement single BB stack-move optimization which...

Revert "Revert "[MemCpyOpt] implement single BB stack-move optimization which unify the static unescaped allocas""

This reverts commit 36a6eb7d.

[MemCpyOpt] check that load/store and dest/src alloca are all in the same bb

Differential Revision: https://reviews.llvm.org/D153453


Co-authored-by: serge-sans-paille <sguelton@mozilla.com>