This increases the size of register operands to be 16-bit.

Not all bytecodes have wide register variants, so when they are
needed a register translator will copy them into a small area
reserved at the top of the 8-bit register range and these registers
are supplied as arguments to the bytecode with 8-bit operands.

This is non-intrusive for typical bytecode where the number of
registers is less than 120. For bytecodes with wide register
operands (above the window) their index needs to be translated
to avoid the reserved translation window.

Enables splay.js to run in Octane and a handful of mjsunit tests.

BUG=v8:4280,v8:4675
LOG=NO

Review URL: https://codereview.chromium.org/1613163002

Cr-Commit-Position: refs/heads/master@{#33516}