[FastISel] Fix a potential bug in FastEmitInst_ri

FastEmitInst_ri was constraining the first operand without checking if it is
a virtual register. Use constrainOperandRegClass as all the other
FastEmitInst_* functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216613 91177308-0d34-0410-b5e6-96231b3b80d8