nir/spirv: fix build_mat4_det stack smasher

nir/spirv: fix build_mat4_det stack smasher

When generating a sub-determinate matrix, a 3-element swizzle array was
indexed with clever inline boolean logic.  Unfortunately, when i and j
are both 3, the index overruns the array, smashing the next variable on
the stack.

For 64 bit builds, the alignment of the 3-element unsigned array leaves
32 bits of spacing before the next local variable, hiding this bug.  On
i386, a subcolumn pointer was smashed then dereferenced.