nir: fix constant-folding of 64-bit fpow
We need to do full pow if 64-bit, and we can do fpow() otherwise. Not the other way around.
Fixes: 9076c4e2 ("nir: update opcode definitions for different bit sizes")
We need to do full pow if 64-bit, and we can do fpow() otherwise. Not the other way around.
Fixes: 9076c4e2 ("nir: update opcode definitions for different bit sizes")