[ScopDetect] Do not add loads out of the SCoP to required invariant loads.

Loads before the SCoP are always invariant within the SCoP and
therefore are no "required invariant loads". An assertion failes in
ScopBuilder when it finds such an invariant load.

Fix by not adding such loads to the required invariant load list. This
likely will cause the region to be not considered a valid SCoP.
We may want to unconditionally accept instructions defined before
the region as valid invariant conditions instead of rejecting them.

This fixes a compilation crash of SPEC CPU2006 453.povray's
render.cpp.

llvm-svn: 314636