Split the interesting bits of ConstantFoldLoadThroughGEPConstantExpr

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148786 91177308-0d34-0410-b5e6-96231b3b80d8