[llvm] [InstCombine] Forward memcpy source to load instruction (PR #140249)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw commented:

Consider the following case:
```
  memcpy(dst, src, len);
  %x = load dst
  br %cond, %then, %else
then:
  %y = load dst
```
This patch forwards the memcpy source pointer to `%x`, but not for `%y`. If `%x` cannot be simplified eventually, the resulting IR is:
```
  memcpy(dst, src, len);
  %x = load src
  br %cond, %then, %else
then:
  %y = load dst
```
This regression is common in real-world programs: https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2354#discussion_r2093327709

Can we start in a more conservative way, by ensuring that the load can be CSEed (query `FindAvailableLoadedValue` recursively with forwarded memcpy source), or the memcpy has only one user in MemorySSA?


https://github.com/llvm/llvm-project/pull/140249