remove some old hacky code that tried to infer whether a store

remove some old hacky code that tried to infer whether a store
occured in a pattern, but failed miserably.  The new code works for
any instruction that has a store in its pattern, including all the
x86 mem op mem instructions.

The only target-independent code that uses this is branch folding,
so this won't change anything in practice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45648 91177308-0d34-0410-b5e6-96231b3b80d8