[SelectionDAG] Teach the vector scalarizer about FP conversions.

This adds support for legalization of instructions of the form:

  [fp_conv] <1 x i1> %op to <1 x double>

where fp_conv is one of fpto[us]i, [us]itofp.  This used to assert
because they were simply missing from the vector operand scalarizer.

A similar problem arose in r190830, with trunc instead.

Fixes PR20778.

Differential Revision: http://reviews.llvm.org/D5810