[ms-inline asm] Handle the enumeration of input and output expressions in a

more robust way to address a few FIXMEs.

The initial implementation, r163342, built the IR asm string and then tried to
patch things on the fly without enough context.  Specifically, it didn't skip
mnemonics nor did it track with assembly instruction an expression was related
to.  The new implementation patches the operands and then builds the final
IR string.

llvm-svn: 163756