Unifies the meaning of kRegCount8 and kRegCount16 across bytecodes.
Call and CallJSRuntime had a slightly different use of the register
count operand. From this change forth, register count operands are
always based off of the previous register operand.

BUG=v8:4280,v8:4675
LOG=N

Review URL: https://codereview.chromium.org/1659023002

Cr-Commit-Position: refs/heads/master@{#33707}