Add Support for Vector Fixed-Point Arithmetic Instructions

[kathlenemagnus-mips]

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

• Vector Fixed-Point Arithmetic Instructions: Vector Single-Width Saturating Add and Subtract
• Vector Fixed-Point Arithmetic Instructions: Vector Single-Width Averaging Add and Subtract
• Vector Fixed-Point Arithmetic Instructions: Vector Single-Width Scaling Shift Instructions
• Vector Fixed-Point Arithmetic Instructions: Vector Narrowing Fixed-Point Clip Instructions