For X87 platform, it has the same general register as ia32 and it will spill the
   context to the stack too.

port bfdc22d7 (r29325).

original commit message:

    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=

Review URL: https://codereview.chromium.org/1316583003

Cr-Commit-Position: refs/heads/master@{#30368}