Add Support for Vector Mask Instructions

[kathlenemagnus-mips]

Add vector sequencer and execution support for the vector mask instructions. At least 1 test should be added with an instruction from each category to verify proper sequencing.

• Vector Mask Instructions: Vector count population in mask vcpop.m
• Vector Mask Instructions: vfirst nd-rst-set mask bit
• Vector Mask Instructions: vmsbf.m set-before-rst mask bit
• Vector Mask Instructions: vmsif.m set-including-rst mask bit
• Vector Mask Instructions: vmsof.m set-only-rst mask bit
• Vector Mask Instructions: Vector Iota Instruction
• Vector Mask Instructions: Vector Element Index Instruction