[CodeGenPrepare] Fix the check of the legality of an instruction.
The API expects an ISD opcode, not an IR opcode.
Fixes a regression for R600.

Related to <rdar://problem/15519855>.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201923 91177308-0d34-0410-b5e6-96231b3b80d8