aco: improve waitcnt insertion around loops

aco: improve waitcnt insertion around loops

Do this by repeating processing of loops until no progress is made.

Totals from affected shaders:
SGPRS: 162576 -> 162576 (0.00 %)
VGPRS: 145228 -> 145228 (0.00 %)
Spilled SGPRs: 668 -> 668 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 15778640 -> 15771336 (-0.05 %) bytes
LDS: 146 -> 146 (0.00 %) blocks
Max Waves: 6087 -> 6087 (0.00 %)

v2: use block_kind_loop_header/block_kind_loop_exit to repeat at the end
    of loops instead of at each continue

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>