[x86] form broadcast of scalar memop even with >1 use

The unseen logic diff occurs because MayFoldLoad() is defined like this:

static bool MayFoldLoad(SDValue Op) {
  return Op.hasOneUse() && ISD::isNormalLoad(Op.getNode());
}

The test diffs here all seem ok to me on screen/paper, but it's hard to know
if that will lead to universally better perf for all targets. For example,
if a target implements broadcast from mem as multiple uops, we would have to
weigh the potential reduction of instructions and register pressure vs.
possible increase in number of uops. I don't know if we can make a truly
informed decision on this at compile-time.

The motivating case that I'm looking at in PR42024:
https://bugs.llvm.org/show_bug.cgi?id=42024
...resembles the diff in extract-concat.ll, but we're not going to change the
larger example there without at least 1 other fix.

Differential Revision: https://reviews.llvm.org/D74088

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