[turbofan] Properly initialize mutable heap numbers in literals.

[turbofan] Properly initialize mutable heap numbers in literals.

We accidently drop the effect of the initializing store on the floor and
just hang on to the effect of the mutable heap number allocation. This
means that the store is actually dead code and will be trimmed from the
graph.

R=mstarzinger@chromium.org
BUG=chromium:610228
NOPRESUBMIT=true
NOTRY=true
LOG=n

[Michael Starzinger, 2016-05-12 11:57:16]

LGTM.

[Benedikt Meurer, 2016-05-12 12:02:32]

Committed patchset #1 (id:1) to pending queue manually as
60a6658f20fb07082e194bec74857af6f2b7b164.