shader/conversion: Fix F2F rounding operations with different sizes

Rounding operations only matter when the conversion size of source and
destination is the same, i.e. .F16.F16, .F32.F32 and .F64.F64.

When there is a mismatch (.F16.F32), these bits are used for IEEE
rounding, we don't emulate this because GLSL and SPIR-V don't support
configuring it per operation.