ac: make use of if/loop build helpers

These helpers insert the basic block in the same order as they
appear in NIR making it easier to follow LLVM IR dumps. The helpers
also insert more useful labels onto the blocks.

TGSI use the line number of the corresponding opcode in the TGSI
dump as the label id, here we use the corresponding block index
from NIR.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 99cdc019)