`asuint` should be able to take vectors in addition to scalar values.
Previously, it would be lowered as a bitcast from the input value to a
vector of uints with a width of 2, which is not large enough if the
input value is larger than a scalar value. In order to handle, for
example, an input value that is a `double4`, we instead perform a
component-wise bitcast.

Fixes microsoft#6735