[AMDGPU] Disable nested endcf collapse

The assumption is that conditional regions are perfectly nested
and a mask restored at the exit from the inner block will be
completely covered by a mask restored in the outer.

It turns out with our current structurizer this is not always
the case.

Disable the optimization for now, but I want to keep it around
for a while to either try after further structurizer changes or
to move it into control flow lowering where we have more info
and reuse the test.

Differential Revision: https://reviews.llvm.org/D75958