Cleanup ImmToIdxMap and noImmForm in PPCRegisterInfo

ImmToIdxMap should be a DenseMap (not a std::map) because there
is no ordering requirement. Also, we don't need a separate list
of instructions for noImmForm in eliminateFrameIndex, because this
list is essentially the complement of the keys in ImmToIdxMap.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178450 91177308-0d34-0410-b5e6-96231b3b80d8