glsl: Rework lowering of non-constant array indexing

glsl: Rework lowering of non-constant array indexing

The previous implementation could easily get tricked if the LHS of an
assignment included a non-constant index that was "inside" another
dereference.  For example:

    mat4 m[2];
    m[0][i] = vec4(0.0);

Due to the way it tracked whether the array was being assigned, it
would think that the non-constant index was in an r-value.  The new
code fixes that by tracking l-values and r-values differently.  The
index is also replaced by cloning the IR and replacing the index
variable instead of the odd way it was done before.

v2: Apply some simplifications suggested by Eric Anholt.  Making
assignment_generator::rvalue be ir_dereference instead of ir_rvalue
simplified the code a bit.

Fixes i965 piglit fs-temp-array-mat[234]-index-wr and
vs-varying-array-mat[234]-index-wr.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34691
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 1731ac308631138ca98d34e8b7070c6e3f981939)

To make bisects work, this also squashes in:

glsl: Correctly return progress from lower_variable_index_to_cond_assign

lower_variable_index_to_cond_assign runs until it can't make any more
progress.  It then returns the result of the last pass which will
always be false.  This caused the lowering loop in
_mesa_ir_link_shader to end before doing one last round of
lower_if_to_cond_assign.  This caused several if-statements (resulting
from lower_variable_index_to_cond_assign) to be left in the IR.

In addition to this change, lower_variable_index_to_cond_assign should
take a flag indicating whether or not it should even generate
if-statements.  This is easily controlled by
switch_generator::linear_sequence_max_length.  This would generate
much better code on architectures without any flow contol.

Fixes i915 piglit regressions glsl-texcoord-array and
glsl-fs-vec4-indexing-temp-src.

Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit c1e591eed41b45c0fcf1dcac8b1b8aaeb6237a38)