There was an assumption that if the instruction had non-native float
as a source, the first source would have such type. This doesn't
hold for Select, and the code failed in two ways
- The boolean source of Select was being converted to the non-native
float type.
- The loop that resolves the bit-size for unsized operands would
trip at `assert(i == 0)` because Select has more than one source.
Re-organize the code to track the types of the sources independently,
and fix both issues above.
Fixes: 90e1b12890 ("spirv: Add bfloat16 support to SpecConstantOp")
Fixes: 51d3c4c889 ("spirv: support float8 spec constant op")
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40157>
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