spirv: Handle doubles when multiplying a mat by a scalar

spirv: Handle doubles when multiplying a mat by a scalar

The code to handle mat multiplication by a scalar tries to pick either
imul or fmul depending on whether the matrix is float or integer.
However it was doing this by checking whether the base type is float.
This was making it choose the int path for doubles (and presumably
float16s).

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>