i965/fs: Clear result before visiting shadow comparitor and LOD info.

i965/fs: Clear result before visiting shadow comparitor and LOD info.

Commit 53c89c67f33639afef951e178f93f4e29acc5d53 ("i965: Avoid generating
MOVs for assignments of expressions.") added the line "this->result =
reg_undef" all over the code.  Unfortunately, since Eric developed his
patch before I landed Ivybridge support, he missed adding it to
fs_visitor::emit_texture_gen7() after rebasing.

Furthermore, since I developed TXD support before Eric's patch, I
neglected to add it to the gradient handling when I rebased.

Neglecting to set this causes the visitor to use this->result as storage
rather than generating a new temporary.  These missing statements
resulted in the same register being used to store several different
values.

Fixes the following piglit tests on Ivybridge:
- glsl-fs-shadow2dproj.shader_test
- glsl-fs-shadow2dproj-bias.shader_test

NOTE: This is a candidate for the 7.11 branch.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>