Optimize TransformationMatrix::multiply() for x86_64
https://bugs.webkit.org/show_bug.cgi?id=105719

Reviewed by Sam Weinig.

On x86_64, we have access to 16 XMM registers. This can hold 32 double values.
We can use that in two ways to optimize matrix multiplications:
-Keep the source matrix completely in registers. Write the result directly in
 the source matrix's memory. This avoids the memcpy at the end of the multiplication
 and various memory operations.
-Use SIMD with SSE to perform 2 operations at a time.

The parameter from the second matrix are loaded one by one in XMM registers.
Loading them with SSE then shuffling the values perform worse than loading
one by one.

This is only enabled on 64bits as x86 only has access to 8 XMM registers and
the function should be written differently.

On a i5, TransformationMatrix::multiply() perform about 3 times faster with the change.

* platform/graphics/transforms/TransformationMatrix.cpp:
(WebCore::TransformationMatrix::multiply):
* platform/graphics/transforms/TransformationMatrix.h:
(TransformationMatrix): Fix an incorrect comment. Unify the comment with the cpp file.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138866 268f45cc-cd09-0410-ab3c-d52691b4dbfc