[x64] Compare map instead of value to heap number map in ToStringStub.

[x64] Compare map instead of value to heap number map in ToStringStub.

Fixes a typo introduced earlier, where we compare the value to heap
number map instead of the map loaded previously.

TBR=jarin@chromium.org

Committed: https://crrev.com/02a2580b1636149d0c296591004e28bcd5fb4656
Cr-Commit-Position: refs/heads/master@{#30859}

[Benedikt Meurer, 2015-09-22 06:55:01]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-09-22 06:55:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1355253002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1355253002/1

[commit-bot: I haz the power, 2015-09-22 07:20:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-22 07:20:12]

Patchset 1 (id:??) landed as
https://crrev.com/02a2580b1636149d0c296591004e28bcd5fb4656
Cr-Commit-Position: refs/heads/master@{#30859}