Prevent implicit re-interpret casts between ExtVector and Scalar types.

Previously, in certain cases lax vector conversions could occur between scalar floating-point values and ExtVector types; these conversions would be simple bitcasts.  We need to allow them with other vector types to support some common headers, but we don't need them for ExtVector.  Preventing them here makes them behave like other operations involving scalars and ExtVectors.

llvm-svn: 247643