nir: Optimize double-precision lower_round_even()

nir: Optimize double-precision lower_round_even()

Use the trick of adding and then subtracting 2**52 (52 is the number of
explicit mantissa bits a double-precision floating-point value has) to
implement round-to-even.

Cuts the number of instructions on SKL of the piglit test
fs-roundEven-double.shader_test from 109 to 21.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>