i965/vec4: add opportunistic behaviour to opt_vector_float()

i965/vec4: add opportunistic behaviour to opt_vector_float()

opt_vector_float() transforms several scalar MOV operations to a single
vectorial MOV.

This is done when those MOV covers all the components of the destination
register. So something like:

mov vgrf3.0.xy:D, 0D
mov vgrf3.0.w:D, 1065353216D
mov vgrf3.0.z:D, 0D

is transformed in:

mov vgrf3.0:F, [0F, 0F, 0F, 1F]

But there are cases where not all the components are written. For
example, in:

mov vgrf2.0.x:D, 1073741824D
mov vgrf3.0.xy:D, 0D
mov vgrf3.0.w:D, 1065353216D
mov vgrf4.0.xy:D, 1065353216D
mov vgrf4.0.w:D, 0D
mov vgrf6.0:UD, u4.xyzw:UD

Nor vgrf3 nor vgrf4 .z components are written, so the optimization is
not applied.

But it could be applied anyway with the components covered, using a
writemask to select the ones written. So we could transform it in:

mov vgrf2.0.x:D, 1073741824D
mov vgrf3.0.xyw:F, [0F, 0F, 0F, 1F]
mov vgrf4.0.xyw:F, [1F, 1F, 0F, 0F]
mov vgrf6.0:UD, u4.xyzw:UD

This commit does precisely that: opportunistically apply
opt_vector_float() when possible.

total instructions in shared programs: 7124660 -> 7114784 (-0.14%)
instructions in affected programs: 443078 -> 433202 (-2.23%)
helped: 4998
HURT: 0

total cycles in shared programs: 64757760 -> 64728016 (-0.05%)
cycles in affected programs: 1401686 -> 1371942 (-2.12%)
helped: 3243
HURT: 38

v2: change vectorize_mov() signature (Matt).
v3: take in account predicates (Juan).
v4 [mattst88]: Update shader-db numbers. Fix some whitespace issues.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>