Fixing an infinite loop problem in InstCombine

Patch by Mohammad Fawaz

This issues started happening after
https://github.com/llvm/llvm-project/commit/b373b5990d5991a920c421b21a352e4ccf4c4993
Basically, if the memcpy is volatile, the collectUsers() function should
return false, just like we do for volatile loads.

Differential Revision: https://reviews.llvm.org/D106950