[x86] use more phadd for reductions

This is part of what is requested by PR42023:
https://bugs.llvm.org/show_bug.cgi?id=42023

There's an extension needed for FP add, but exactly how we would specify
that using flags is not clear to me, so I left that as a TODO.
We're still missing patterns for partial reductions when the input vector
is 256-bit or 512-bit, but I think that's a failure of vector narrowing.
If we can reduce the widths, then this matching should work on those tests.

Differential Revision: https://reviews.llvm.org/D64760

llvm-svn: 366268