Add an IR folding rule to convert Add32(x,x) into Shl32(x,1). This

Add an IR folding rule to convert Add32(x,x) into Shl32(x,1).  This
fixes #118466 and it also gets rid of a bunch of false positives for
KDE 3.5.2 built by gcc-4.0.2 on x86, of the form shown below.

  Use of uninitialised value of size 4
    at 0x4BFC342: QIconSet::pixmap(QIconSet::Size, QIconSet::Mode,
                                   QIconSet::State) const (qiconset.cpp:530)
    by 0x4555BE7: KToolBarButton::drawButton(QPainter*)
                  (ktoolbarbutton.cpp:536)
    by 0x4CB8A0A: QButton::paintEvent(QPaintEvent*) (qbutton.cpp:887)

git-svn-id: svn://svn.valgrind.org/vex/trunk@1617