[JSC] Use (x + x) instead of (x * 2) when possible

[JSC] Use (x + x) instead of (x * 2) when possible
https://bugs.webkit.org/show_bug.cgi?id=148051

Patch by Benjamin Poulain <bpoulain@apple.com> on 2015-08-16
Reviewed by Michael Saboff.

When multiplying a number by 2, JSC was loading a constant "2"
in register and multiplying it with the first number:

    mov $0x4000000000000000, %rcx
    movd %rcx, %xmm0
    mulsd %xmm0, %xmm1

This is a problem for a few reasons.
1) "movd %rcx, %xmm0" only set half of XMM0. This instruction
   has to wait for any preceding instruction on XMM0 to finish
   before executing.
2) The load and transform itself is large and unecessary.

To fix that, I added a StrengthReductionPhase to transform
multiplications by 2 into a addition.

Unfortunately, that turned the code into:
    movsd %xmm0 %xmm1
    mulsd %xmm1 %xmm0

The reason is GenerationInfo::canReuse() was not accounting
for nodes using other nodes multiple times.

After fixing that too, we now have the multiplications by 2
done as:
    addsd %xmm0 %xmm0

* dfg/DFGGenerationInfo.h:
(JSC::DFG::GenerationInfo::useCount):
(JSC::DFG::GenerationInfo::canReuse): Deleted.
* dfg/DFGSpeculativeJIT.cpp:
(JSC::DFG::FPRTemporary::FPRTemporary):
* dfg/DFGSpeculativeJIT.h:
(JSC::DFG::SpeculativeJIT::canReuse):
(JSC::DFG::GPRTemporary::GPRTemporary):
* dfg/DFGStrengthReductionPhase.cpp:
(JSC::DFG::StrengthReductionPhase::handleNode):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@188519 268f45cc-cd09-0410-ab3c-d52691b4dbfc