Subtract the lower single-precision (32-bit) floating-point element in b from the lower single-precision (32-bit)
floating-point element in a, store the subtration result in the lower element of result, and copy the upper 3
packed elements from a to the upper elements of result.
Subtract the lower single-precision (32-bit) floating-point element in b from the lower single-precision (32-bit) floating-point element in a, store the subtration result in the lower element of result, and copy the upper 3 packed elements from a to the upper elements of result.