i965/vs/skl: Use vec4 datatypes for message header

i965/vs/skl: Use vec4 datatypes for message header

We're using a SIMD4x2 sampler message, which has execsize 4, and so the
register width must be <= 4.  Use <4,4,1> regioning instead of <8,8,1>
regioning to access the same data but avoid tripping the assert.

Fixes the following piglit tests:
spec/glsl-1.20/compiler/structure-and-array-operations/array-selection.vert
spec/glsl-es-3.00/compiler/uniform_block/interface-name-basic.vert
spec/glsl-es-3.00/compiler/uniform_block/interface-name-field-clashes-with-struct.vert
spec/glsl-es-3.00/compiler/uniform_block/interface-name-field-clashes-with-function.vert
spec/glsl-es-3.00/compiler/uniform_block/interface-name-array.vert
glslparsertest/glsl2/condition-07.vert
spec/glsl-es-3.00/compiler/uniform_block/interface-name-field-clashes-with-variable.vert

v2: Better commit message courtesy of Ken.
I had a discussion with Ken, and we both question how we end up with a mov and
execsize 4. For now though, this fixes the piglit tests, so we can worry about
it later.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>