Remove StringMap::GetOrCreateValue in favor of StringMap::insert

Having two ways to do this doesn't seem terribly helpful and
consistently using the insert version (which we already has) seems like
it'll make the code easier to understand to anyone working with standard
data structures. (I also updated many references to the Entry's
key and value to use first() and second instead of getKey{Data,Length,}
and get/setValue - for similar consistency)

Also removes the GetOrCreateValue functions so there's less surface area
to StringMap to fix/improve/change/accommodate move semantics, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222319 91177308-0d34-0410-b5e6-96231b3b80d8
[SeparateConstOffsetFromGEP] Allow SeparateConstOffsetFromGEP pass to lower GEPs. 
If LowerGEP is enabled, it can lower a GEP with multiple indices into GEPs with a single index
or arithmetic operations. Lowering GEPs can always extract structure indices. Lowering GEPs can
also give use more optimization opportunities. It can benefit passes like CSE, LICM and CGP.

Reviewed in http://reviews.llvm.org/D5864


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222328 91177308-0d34-0410-b5e6-96231b3b80d8