glsl: fix infinite loop caused by bug in loop unrolling pass

Just checking for 2 jumps is not enough to be sure we can do a
complex loop unroll. We need to make sure we also have also found
2 loop terminators.

Without this we were attempting to unroll a loop where the second
jump was nested inside multiple ifs which loop analysis is unable
to detect as a terminator. We ended up splicing out the first
terminator but failed to actually unroll the loop, this resulted
in the creation of a possible infinite loop.

Fixes: 646621c6 "glsl: make loop unrolling more like the nir unrolling path"

Tested-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105670
(cherry picked from commit 56b86739)