Especially when loading and untagging SMIs within code stubs, instances of the following pattern appear in the generated code:

movq %rax,[%rbx+%rcx*4+0xf]
sarq %rax, 32

This CL changes that code to:

movsxlq %rax, [%ebx+%ecx*4+0x13]

BUG=

Review URL: https://codereview.chromium.org/1845043004

Cr-Commit-Position: refs/heads/master@{#35194}