Add a "Register Length" tag to vector registers

[mbitsnbites]

Functionality

• Writing to a vector register updates its Register Length to the operation Vector Length.
• In most instructions the Register Length of the first soruce vector operand defines the operation Vector Length.
• The VL register defines the operation Vector Length for certain "register initialization" instructions, e.g:
  • Stride based memory loads (LDW, LDUB etc) and load effective address (LDEA).
  • Operations using the VZ register as the first operand (e.g. OR V2, VZ, R7).
• Elements with an index >= RL are read as zero.
• Clearing a vector register to RL=0 marks the register as "unused", which can reduce context switch overhead.

New instructions

• A new "get length" instruction can be used for querying the Register Length of a vector register.
• Possibly add a "clear register" instruction that sets RL to zero for one register (or several registers at once?).
• Possibly add a "set length" instruction.