vkd3d-shader/ir: Remove remaining instances of instruction preallocation.
I recently realized that after !1761 (merged) there are still some calls to shader_instruction_array_reserve(), but in ir.c.
We have to get rid of these too if we want to replace the vkd3d_shader_instruction_array with a data type that doesn't require instruction preallocation. I rebased my rbseq implementation on top of these patches and removed this preallocation functionality in my rbseq6 branch.