X86WinAllocaExpander: Drop code looking through register copies (PR41786)

This code was never covered by tests, in PR41786 it was pointed out that
the deletion part doesn't work, and in a full Chrome build I was never
able to hit the code path that looks through copies. It seems the
situation it's supposed to handle doesn't actually come up in practice.

Delete it to simplify the code.

Differential revision: https://reviews.llvm.org/D61671

llvm-svn: 360320