[X86] Allow load folding into PUSH instructions

[X86] Allow load folding into PUSH instructions

Adds pushes to the folding tables.
This also required a fix to the TD definition, since the memory forms of
the push instructions did not have the right mayLoad/mayStore flags.

Differential Revision: http://reviews.llvm.org/D11340

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243010 91177308-0d34-0410-b5e6-96231b3b80d8