[XLA] Fix handling of in-place dynamic-update-slice in HloDataflowAnalysis.

The code replaced was trying to determine whether it's safe to do an in-place
dynamic-update-slice.  It had two bugs.

1. It was assuming that index 1/2 of dynamic-slice/dynamic-update-slice
contained all of the slice indices.  This used to be true, back when slice
indices were an array, but the new scalar-indices form of these instructions
can have many index operands.  Only checking one operand is busted.

2. It was checking that all uses of the relevant operand were by a
dynamic-update-slice op, but it wasn't checking that this user *was in fact the
root*.  So it was possible to create a fusion with two DUS instructions, one of
which was the root, and another of which was not, and confuse this
pattern-matcher.

PiperOrigin-RevId: 236254299