Convert the lower single-precision (32-bit) floating-point element in b to a double-precision (64-bit) floating-point element, store that in the lower element of result, and copy the upper element from a to the upper
See Implementation
Convert the lower single-precision (32-bit) floating-point element in b to a double-precision (64-bit) floating-point element, store that in the lower element of result, and copy the upper element from a to the upper