Move some constant folding logic for loads out of instcombine into

Move some constant folding logic for loads out of instcombine into
Analysis/ConstantFolding.cpp.  This doesn't change the behavior of
instcombine but makes other clients of ConstantFoldInstruction
able to handle loads.  This was partially extracted from Eli's patch
in PR3152.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84836 91177308-0d34-0410-b5e6-96231b3b80d8