i965/fs: Get rid of fs_inst::set_smear().

i965/fs: Get rid of fs_inst::set_smear().

component() was generally a better alternative because of several
issues set_smear() had:

 - It wouldn't take the original stride and offset of the register
   into account, which means that set_smear() on the result of
   e.g. another set_smear() call or an offset() call would give a
   bogus region as result.

 - It was an inherently destructive operation.  See the
   'nir_intrinsic_shader_clock' hunk below for how this could lead to
   subtle bugs in cases where set_smear() was called multiple times on
   the same register like 'r.set_smear(0), r.set_smear(1)' with the
   expectation that each call would return a separate value instead of
   a reference to the same subsequently mutated object.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>