Split the interesting bits of ConstantFoldLoadThroughGEPConstantExpr

out into a new ConstantFoldLoadThroughGEPIndices (more useful) function
and rewrite it to be simpler, more efficient, and to handle the new
ConstantDataSequential type.

Enhance ConstantFoldLoadFromConstPtr to handle ConstantDataSequential.

llvm-svn: 148786