Make sure that top bits are zero when storing untagged 32 bit values

Make sure that top bits are zero when storing untagged 32 bit values
in 64 bit spill slots.
Committed: https://code.google.com/p/v8/source/detail?r=10774

[ulan, 2012-02-10 16:45:57]

Drive by comment. I think processor already clears out the top bits for 32-bit
MOVs.

Intel manual Vol. 1 3-17:

When in 64-bit mode, operand size determines the number of valid bits in the
destination general-purpose register:
- 64-bit operands generate a 64-bit result in the destination general-purpose
register.
- 32-bit operands generate a 32-bit result, zero-extended to a 64-bit result in
the destination general-purpose register.

[Erik Corry, 2012-02-13 20:18:27]

Thanks for pointing that out.  New version uploaded.

[Vyacheslav Egorov (Chromium), 2012-02-15 20:35:58]

lgtm