[DAGCombiner] Fix invalid size request in combineRepeatedFPDivisors

If we have a vector FP division with a splatted divisor, use
getVectorMinNumElements when scaling the num of uses by splat factor.

For AArch64 the combine kicks in for the <vscale x 4 x float> case since it's
above the fdiv threshold (3) when scaling num uses by splat factor, but the
codegen is worse (splat + vector fdiv + vector fmul) than the <vscale x 2 x
double> case (splat + vector fdiv).

If the combine could be converted into a scalar FP division by
scalarizeBinOpOfSplats it may be cheaper, but it looks like this is predicated
on the isExtractVecEltCheap TLI function which is implemented for x86 but not
AArch64. Perhaps for now combineRepeatedFPDivisors should only scale num uses
by splat if the division can be converted into scalar op.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D118343