Only basic things for now, but enough, for example, to emit

mov rax, [rax+0xc]

instead of

mov rax, 0x3
mov rdx, [rdx+rax*4]

on x64.

BUG=

Review-Url: https://codereview.chromium.org/2211633003
Cr-Commit-Position: refs/heads/master@{#38338}