Fix the last of the major PPC GEP folding deficiencies. This will allow

Fix the last of the major PPC GEP folding deficiencies.  This will allow
the ISel to use indexed and non-zero immediate offsets for GEPs that have
more than one use.  This is common for instruction sequences such as a load
followed by a modify and store to the same address.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16493 91177308-0d34-0410-b5e6-96231b3b80d8