[InstSimplify] try constant folding intrinsics before general simplifications

This matches the behavior of simplify calls for regular opcodes -
rely on ConstantFolding before spending time on folds with variables.

I am not aware of any diffs from this re-ordering currently, but there was
potential for unintended behavior from the min/max intrinsics because that
code is implicitly assuming that only 1 of the input operands is constant.