Fix a missing MemorySSA update in breakLoopBackedge

This is a case I'd missed in 6a8237. The odd bit here is that missing the edge removal update seems to produce MemorySSA which verifies, but is still corrupt in a way which bothers following passes. I wasn't able to reduce a single pass test case, which is why the reported test case is taken as is.

Differential Revision: https://reviews.llvm.org/D109068