This patch extends https://reviews.llvm.org/D153453 to muti-BB case, aims to remove memcpy like https://internals.rust-lang.org/t/the-way-to-see-how-memcpy-and-alloca-introduced-by-move-is-optimized/19148/4
Single-BB version was like the following
(1) The src is fully copied to dest by memcpy, memmove or store&load.
(2) src and dest are 1) static alloca, which is to avoid stacksave/stackrestore handling, 2) unescaped allocas.
(3) The dest has no Mod Ref except full-sized lifetime intrinsic and copy itself, from the alloca to the store.
(4) If the dest has no Mod => src has no Ref, dest has no Ref => src has no Mod from the store to the end of BB.
This patch changes the original single-BB patch's ModRef checking conditions (3) and (4) by using reachability and dominator tree query as the following
(3) Mod/Ref for dest doesn't exist if it's reachable to full-size copy
(4) All uses not post dominated by the full-size copy satisfies (dest-Mod NAND src-Ref) AND (dest-Ref NAND src-Mod),
pre-commit test: https://reviews.llvm.org/D155422
compile-time regression: https://llvm-compile-time-tracker.com/compare.php?from=2975ccb4b06b3d3aedd86ab21729146e441521d7&to=83fcffd325e79dd89e2b932b053945f868659f56&stat=instructions:u