r300: don't move position output to the end when duplicating it for WPOS

r300: don't move position output to the end when duplicating it for WPOS

Instead just emit both outputs as soon as possible.

If the last write is inside a loop or a branch, emit it after
the ENDLOOP or ENDIF. This saves some temps and also allows us
to potentially benefit from R300_PVS_XYZW_VALID_INST as right
now the position output write is always penultimate with the
WPOS output being the last.

total temps in shared programs: 14101 -> 14029 (-0.51%)
temps in affected programs: 435 -> 363 (-16.55%)
helped: 72
HURT: 0

Signed-off-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Reviewed-by: Filip Gawin <filip.gawin@zoho.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15321>