The current context is stored as a stack slot on the interpreter frame
and therefore we don't need to also maintain a machine register for the
context. Removes this register from bytecode handlers.

In the process modifies this frees up a register on ia32 to keep the
dispatch table pointer in a register rather than on a stack slot on
ia32.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1887493004

Cr-Commit-Position: refs/heads/master@{#35511}