Fix phis for non-sse2 double values

Fix phis for non-sse2 double values

Since hydrogen will try to merge phi-inputs using registers, even if
the values are spilled, we cannot mark LGoto as double clobbering
instruction.

BUG=
R=verwaest@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=16671

[Toon Verwaest, 2013-09-11 12:28:30]

lgtm with comment. Doesn't support two doubles yet, but I think it's ok to leave
it like this until we have an actual use-case for it.

https://codereview.chromium.org/23068038/diff/8001/src/ia32/lithium-codegen-i...
File src/ia32/lithium-codegen-ia32.cc (right):

https://codereview.chromium.org/23068038/diff/8001/src/ia32/lithium-codegen-i...
src/ia32/lithium-codegen-ia32.cc:672: ASSERT(stack_depth_ <= 1);
Add comment as discussed. If ever used for other stubs than the binary op stub,
two pairs of doubles could be joined into two phis, causing a stack-depth of 2.
In that case, either the stack is in the same order in both incoming blocks,
requiring no gap; or the values on the stack need to be swapped in one of the
branches.

[oliv, 2013-09-12 09:05:45]

Committed patchset #2 manually as r16671 (presubmit successful).