glsl: fix lower jumps for returns when loop is inside an if

glsl: fix lower jumps for returns when loop is inside an if

Previously we would just escape the loop and move everything
following the loop inside the if to the else branch of a new if
with a return flag conditional. However everything outside the
if the loop was nested in would still get executed.

Adding a new return to the then branch of the new if fixes this
and we just let a follow pass clean it up if needed.

Fixes:
tests/spec/glsl-1.10/execution/vs-nested-return-sibling-loop.shader_test
tests/spec/glsl-1.10/execution/vs-nested-return-sibling-loop2.shader_test

Cc: "13.0 17.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>