[test][InstCombine] Add a test case for volatile memcpy forwarding in...

[test][InstCombine] Add a test case for volatile memcpy forwarding in InstCombine, which is currently optimized incorrectly.

D136822 demonstrated that we currently delete volatile memcpys in InstCombine,
which we shouldn't do. This commit adds a test for this. A forthcoming commit
will fix it.

Differential Revision: https://reviews.llvm.org/D137029