llvm-mc/AsmMatcher: Switch to a unified function to convert operands to MCInst,

llvm-mc/AsmMatcher: Switch to a unified function to convert operands to MCInst,
so that terminal states are as simple as possible.
 - If we were willing to assume that the order that operands get inserted in the
   MCInst is fixed we could actually dispose with this altogether, although it
   might be nice to have the flexibility to change it later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78458 91177308-0d34-0410-b5e6-96231b3b80d8