Rename nir_op_fpow to nir_op_fpowr
This makes what we do in vtn consistent with the naming in SPIR-V and the IEEE 754 spec. When looking for special case behaviour of nir_op_fpow, you need to look at the powr rules in the IEEE spec, not at the pow rules. Let's rename NIR to make it obvious what's happening.
Actual pow isn't supported in NIR at all, and gets lowered via libclc.
Cc @jekstrand