Passes the current context to bytecode interpreter handlers. This is held in the
context register on all architectures except for ia32 where there are too few
registers and it is instead spilled to the stack.

Also changes Load/StoreRegister to use kMachAnyTagged representation since they
should only ever hold tagged values.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1294133004

Cr-Commit-Position: refs/heads/master@{#30325}