[X86] Rearrange code in X86InstrInfo.cpp to put all the foldMemoryOperandImpl methods together without partial/undef register handling in the middle. NFC

I have a future patch that wants to make use of the one of the partial functions in one of the earlier memory folding methods and the current ordering prevents that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316883 91177308-0d34-0410-b5e6-96231b3b80d8

Author: topperc