nir/compiler: keep same bit size when lowering with flrp

This was probably not caught before because no supported test was exercising the flrp lowering with other bit size different than 32.

With the arrival of VK_KHR_shader_float_controls we will have some of those and, unless we keep the bit size, we will end with something like:

../src/compiler/nir/nir_builder.h:420: nir_builder_alu_instr_finish_and_insert: Assertion `src_bit_size == bit_size' failed.

Signed-off-by: Andres Gomez agomez@igalia.com