glsl: Improve precision of mod(x,y)

Currently, Mesa uses the lowering pass MOD_TO_FRACT to implement
mod(x,y) as y * fract(x/y). This implementation has a down side though:
it introduces precision errors due to the fract() operation. Even worse,
since the result of fract() is multiplied by y, the larger y gets the
larger the precision error we produce, so for large enough numbers the
precision loss is significant. Some examples on i965:

Operation                           Precision error
-----------------------------------------------------
mod(-1.951171875, 1.9980468750)      0.0000000447
mod(121.57, 13.29)                   0.0000023842
mod(3769.12, 321.99)                 0.0000762939
mod(3769.12, 1321.99)                0.0001220703
mod(-987654.125, 123456.984375)      0.0160663128
mod( 987654.125, 123456.984375)      0.0312500000

This patch replaces the current lowering pass with a different one
(MOD_TO_FLOOR) that follows the recommended implementation in the GLSL
man pages:

mod(x,y) = x - y * floor(x/y)

This implementation eliminates the precision errors at the expense of
an additional add instruction on some systems. On systems that can do
negate with multiply-add in a single operation this new implementation
would come at no additional cost.

v2 (Ian Romanick)
- Do not clone operands because when they are expressions we would be
duplicating them and that can lead to suboptimal code.

Fixes the following 16 dEQP tests:
dEQP-GLES3.functional.shaders.builtin_functions.precision.mod.mediump_*
dEQP-GLES3.functional.shaders.builtin_functions.precision.mod.highp_*

Reviewed-by: Ian Romanick <[email protected]>

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