[RISCV][GlobalISel] Select ALU GPR instructions

Some instruction selection patterns required for ALU GPR instructions have already been automatically imported from existing TableGen descriptions - this patch simply adds testing for them. Logic for selecting constants and copies has been added, along with the first of the GIComplexPatternEquiv definitions required to select the shiftMaskXLen ComplexPattern. New patterns have been added to directly select RV64 W instructions from gMIR rather than using custom gMIR operations earlier in the pipeline. In future this could also support the checks present in the DAGToDAGISel for finding ops whose users only require the lower 32 bits.

Differential Revision: https://reviews.llvm.org/D76445