[x86] fix remaining miscompile bug in horizontal binop matching (PR40243)

When we use the partial-matching function on a 128-bit chunk, we must 
account for the possibility that we've matched undef halves of the
original source vectors, so the outputs may need to be reset.

This should allow closing PR40243:
https://bugs.llvm.org/show_bug.cgi?id=40243

llvm-svn: 350830