Teach LegalizeTypes how to expand and promote CTLZ,

CTTZ and CTPOP.  The expansion code differs from
that in LegalizeDAG in that it chooses to take the
CTLZ/CTTZ count from the Hi/Lo part depending on
whether the Hi/Lo value is zero, not on whether
CTLZ/CTTZ of Hi/Lo returned 32 (or whatever the
width of the type is) for it.  I made this change
because the optimizers may well know that Hi/Lo
is zero and exploit it.  The promotion code for
CTTZ also differs from that in LegalizeDAG: it
uses an "or" to get the right result when the
original value is zero, rather than using a compare
and select.  This also means the value doesn't
need to be zero extended.

llvm-svn: 47075