Make each target map all inline assembly memory constraints to InlineAsm::Constraint_...

Make each target map all inline assembly memory constraints to InlineAsm::Constraint_m. NFC.

Summary:
This is instead of doing this in target independent code and is the last
non-functional change before targets begin to distinguish between
different memory constraints when selecting code for the ISD::INLINEASM
node.

Next, each target will individually move away from the idea that all
memory constraints behave like 'm'.

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D8173

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232373 91177308-0d34-0410-b5e6-96231b3b80d8