An integer `+`, `-` or `*` whose operands and result are proven within
the uint32 range, and whose operand is a `>>>` result or arithmetic on
one, such as `((i + j) * (i + j + 1) >>> 1) + i + 1`, is now 32-bit
arithmetic without wrap, and its double converts from the 32 bits
(`uitofp i32`). Vectorized, that conversion takes three instructions on
x86-64 without AVX-512, where the 64-bit sum's unsigned conversion took
six. Sums of counters and literals, such as `i + 1`, keep the 64-bit
form, whose conversion LLVM folds with their uses as indexes.
Chris Tate
3eed91a29b