ir3: fix various spilling bugs

This series fixes various spilling bugs discovered while trying to get IR3_SHADER_DEBUG=spillall working on shader-db and CTS. See the individual commit messages for the specific bugs that were fixed.

This series also contains the introduction of a new ir3_cursor/ir3_builder API, mirroring the API found in NIR for inserting instructions. The reason for introducing this now is that ir3_spill.c contained many functions to create instructions that used an ir3_instruction pointer to indicate where it should be inserted: before the instruction if set, at the end of the block if NULL. One of the bug fixes needed to insert instructions at the beginning of blocks which was impossible to express properly with the current design. Using the new ir3_cursor makes this easy.