[RISCV] Pass OptLevel to `RISCVDAGToDAGISel` correctly

Originally, `OptLevel` isn't passed into the `MachineFunctionPass`.
This lets the default parameter of `SelectionDAGISel`, which is
`CodeGenOpt::Default`, be passed in. OptLevelChanger captures the
optimization level with the parameter, and rather not the value
within `TargetMachine`. This lets the optimization be
unintentionally overwriten if other value than `CodeGenOpt::Default`
passed.

This patch fixes this by passing the optimization level rather
than using the default value.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D126641