Add relocatable pointers for wasm memory references that need to be updated when wasm GrowMemory is used. Code generator changes to accept relocatable constants as immediates.

R=titzer@chromium.org, yangguo@chromium.org, bradnelson@chromium.org

Committed: https://crrev.com/eb5fe0df64ec0add423b2a1f6fb62d5a33dce2a5
Cr-Commit-Position: refs/heads/master@{#35182}

Committed: https://crrev.com/297932a302ce0b73c3618ef9e4eba9d9d241f2b3
Cr-Commit-Position: refs/heads/master@{#35400}

Review URL: https://codereview.chromium.org/1759383003

Cr-Commit-Position: refs/heads/master@{#35407}