i965/vs: Fix bogus assertion in emit_block_move()

i965/vs: Fix bogus assertion in emit_block_move()

i965 processes assignments of whole structures using
vec4_visitor::emit_block_move, a recursive function which visits each
element of a structure or array (to arbitrary nesting depth) and
copies it from the source to the destination.  Then it increments the
source and destination register numbers so that further recursive
invocations will copy the rest of the structure.  In addition, it sets
the swizzle field for the source register to an appropriate value of
swizzle_for_size(...) for the size of each element being copied, so
that later optimization passes won't be fooled into thinking that
unused vector elements are live.

This all works fine.  However, emit_block_move also contains an
assertion to verify, before setting the swizzle field for the source
register, that the source register doesn't already contain a
nontrivial swizzle.  The intention is to make sure that the caller of
emit_block_move hasn't already done some swizzling of the data before
the call, which emit_block_move would then counteract when it
overwrites the swizzle field.  But the assertion is at the lowest
level of nesting of emit_block_move, which means that after the first
element is copied, instead of checking the swizzle field set by the
caller, it checks the swizzle field used when moving the previous
element.  That means that if the structure contains elements of
different vector sizes (which therefore require different swizzles),
the assertion will erroneously fire.

This patch moves the assertion from emit_block_move to the calling
function, vec4_visitor::visit(ir_assignment *).  Since the caller is
non-recursive, the assertion will only happen once, and won't be
fooled by emit_block_move's modification of the swizzle field.

This patch also reverts commit fe006a7 (i965/vs: Fix swizzle related
assertion), which attempted to fix the bug by making the assertion
more lenient, but only worked properly for structures, arrays, and
matrices in which each constituent vector is the same size.

This fixes the problem described in comment 9 of
https://bugs.freedesktop.org/show_bug.cgi?id=40865.  Unfortunately, it
doesn't fix the whole bug, since the test in question is also failing
due to lack of register spilling support in the VS.

Fixes piglit test vs-assign-varied-struct.  No piglit regressions on
Sandy Bridge.

This is a candidate for the 8.0 release branch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=40865#c9
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>