[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.