[DAGTypeLegalizer] Handle SIGN/ZERO_EXTEND in WidenVecRes_Convert().

In case of a SIGN/ZERO_EXTEND of an incomplete vector type (using only a
partial number of available vector elements), WidenVecRes_Convert() used to
resort to scalarization.

This patch adds a handling of the (common) case where an input vector can be
found of same width as the widened result vector, by converting the node to
SIGN/ZERO_EXTEND_VECTOR_INREG.

Review: Eli Friedman
llvm-svn: 293268