i965/gs: Fixup gl_PointSize on entry to geometry shaders.

i965/gs: Fixup gl_PointSize on entry to geometry shaders.

gl_PointSize is stored in the w component of VARYING_SLOT_PSIZ, but
the geometry shader infrastructure assumes that it should look for all
geometry shader inputs of type float in the x component.  So when
compiling a geomtery shader that uses a gl_PointSize input, fix it up
during the shader prolog by moving the w component to the x component.

This is similar to how we emit fixups and workarounds for vertex
shader attributes.

Fixes piglit test spec/glsl-1.50/execution/geometry/core-inputs.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>