Port 28e3467a (r38361)

  original commit message:
    Adding new methods to the code stub assembler and interpreter
    assembler to combine loading and untagging SMIs, so that on 64-bit
    architectures we can avoid loading the full 64 bits and load the
    32 interesting bits directly instead.

BUG=

Review-Url: https://codereview.chromium.org/2265043002
Cr-Commit-Position: refs/heads/master@{#38811}