[Hexagon] Avoid crash on CONCAT_VECTORS with illegal element types

Legal vector element types may not be legal as scalar types. When
CONCAT_VECTORS is converted to BUILD_VECTOR, the individual vector
elements become standalone operands to the build operation. If they
have illegal (scalar) types, they need to be made legal. In doing
so, the case of TRUNCATE was not handled, causing an assertion to
fail.