[TTI][X86] getMinMaxCost - use existing float minnum/maxnum intrinsic cost...

[TTI][X86] getMinMaxCost - use existing float minnum/maxnum intrinsic cost values instead of maintaining a duplicate cost table

Without fastmath (nnan) flags, minnum/maxnum must perform isnan handling as well as fmin/fmax - meaning the costs are notably higher, this is correctly handled in getIntrinsicInstrCost but was missing from the getMinMaxCost cost tables (which assumed fastmath).

Followup to 63c38953 which handled the integer cases