Move instruction flag inference out of InstrInfoEmitter and into

Move instruction flag inference out of InstrInfoEmitter and into
CodeGenDAGPatterns, where it can be used in other tablegen backends.
This allows the inference to be done for DAGISelEmitter so that it
gets accurate mayLoad/mayStore/isSimpleLoad flags.

This brings MemOperand functionality back to where it was before
48329. However, it doesn't solve the problem of anonymous patterns
which expand to code that does loads or stores.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49123 91177308-0d34-0410-b5e6-96231b3b80d8