[MachineCombiner][RISCV] Relax optimization level requirement

Enable Machine Combiner for O1/O2/O3 optimization levels. It makes RISCV
consistent with other targets running Machine Combiner.

Originally it was enabled only for -O3, however I looked through time reports
and usually it takes 0.1%-0.4% of total time, and never takes more than 1.0%.

Differential Revision: https://reviews.llvm.org/D136339