ac: convert nir_op_f2f32 src to a float

ac: convert nir_op_f2f32 src to a float

Fixes the following piglit test:

./bin/arb_vertex_attrib_64bit-check-explicit-location -auto -fbo

Where we would end up with the nir such as:

	vec1 64 ssa_11 = pack_64_2x32_split ssa_9, ssa_10
	vec1 32 ssa_12 = f2f32 ssa_2

And our pack_64_2x32_split nir to llvm code always produces
a 64bit integer as output.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>