nir: Walk blocks in source code order in lower_vars_to_ssa.

Prior to this commit rename_variables_block() is recursively called,
performing a depth-first traversal of the control flow graph. The
function uses a non-trivial amount of stack space for local variables,
which puts us in danger of smashing the stack, given a sufficiently deep
dominance tree.

XCOM: Enemy Within contains a shader with such a dominance tree (1574
nir_blocks in total, depth of at least 143).

Jason tells me that he believes that any walk over the nir_blocks that
respects dominance is sufficient (a DFS might have been necessary prior
to the introduction of nir_phi_builder).

In fact, the introduction of nir_phi_builder made the problem worse:
rename_variables_block(), walks to the bottom of the dominance tree
before calling nir_phi_builder_value_get_block_def() which walks back to
the top of the dominance tree...

In any case, this patch ensures we avoid that problem as well.

Cc: [email protected]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97225
Reviewed-by: Connor Abbott <[email protected]>

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