Various failures for MIPS were introduced by the refactoring of
the way frames are marked https://codereview.chromium.org/1696043002.

Problems were caused during context loading in CheckAccessGlobalProxy
* The value of the register at was unintentionally modified.
* Use of branch instruction instead of branch macro resulted in
  a branch instruction in forbidden slot.

BUG=

Review URL: https://codereview.chromium.org/1812463002

Cr-Commit-Position: refs/heads/master@{#34829}