aco: emit IR in IF's merge block instead if the other side ends in a jump

aco: emit IR in IF's merge block instead if the other side ends in a jump

Fixes NIR such as:
if (divergent) {
   a = sgpr()
} else {
   break;
}
use(a)

Previously we would have emitted:
if (divergent) {
   a = sgpr()
}
if (!divergent) {
   break;
}
use(a)

But "a" isn't available at it's use. Now we emit:
if (divergent) {
}
if (!divergent) {
   break;
}
a = sgpr()
use(a)

pipeline-db (Navi):
Totals from affected shaders:
SGPRS: 1936 -> 1936 (0.00 %)
VGPRS: 1264 -> 1264 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 159408 -> 159152 (-0.16 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 81 -> 81 (0.00 %)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
CC: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2557
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3658>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3658>