[GlobalISel] fdiv to fmul transform (#144305)
This is a port of the SDAG DAGCombiner::combineRepeatedFPDivisors
combine that looks for multiple fdiv operations with the same divisor
and converts them to a single reciprocal fdiv and multiple fmuls. It is
currently a fairly faithful port, with some additions to make sure that
the newly created fdiv dominates all new uses. Compared to the SDAG
version it also drops some logic about splat uses which assumes no
vector fdivs and some logic about x/sqrt(x) which does not yet apply to
GISel.
