[DAGCombiner] skip reciprocal divisor optimization for x/sqrt(x), better

I tried to fix this in:
rG716e35a0cf53
...but that patch depends on the order that we encounter the
magic "x/sqrt(x)" expression in the combiner's worklist.

This patch should improve that by waiting until we walk the
user list to decide if there's a use to skip.

The AArch64 test reveals another (existing) ordering problem
though - we may try to create an estimate for plain sqrt(x)
before we see that it is part of a 1/sqrt(x) expression.