This value should not be directly interpreted. Also avoids
a function only used for an assert.

This patch fixes:

  llvm/lib/Target/AMDGPU/GCNDPPCombine.cpp:298:9: error: unused
  variable 'Src0Idx' [-Werror,-Wunused-variable]

It is not safe to use isOperandLegal on an instruction that does
not have a complete set of operands. Unforunately the APIs are
not set up in a convenient way to speculatively check if an instruction
will be legal in a hypothetical instruction. Build all the operands
and then verify they are legal after. This is clumsy, we should have
a more direct check for will these operands give a legal instruction.

This seems to fix a missed optimization in the gfx11 test. The
fold was firing for gfx1150, but not gfx1100. Both should support
vop3 literals so I'm not sure why it wasn't working before.

This is NFCI at this point.

We only emits v_mov_b32/64_dpp. Don't combine t16 instructions with mov
dpp. Update the test inputs to be legal.

It is future work to emit v_mov_b16_dpp, and then update GCNDPPCombine
to combine it with the 16-bit instructions.

- Change InstrInfoEmitter to emit OpName as an enum class
  instead of an anonymous enum in the OpName namespace.
- This will help clearly distinguish between values that are 
  OpNames vs just operand indices and should help avoid
  bugs due to confusion between the two.
- Rename OpName::OPERAND_LAST to NUM_OPERAND_NAMES.
- Emit declaration of getOperandIdx() along with the OpName
  enum so it doesn't have to be repeated in various headers.
- Also updated AMDGPU, RISCV, and WebAssembly backends
  to conform to the new definition of OpName (mostly
  mechanical changes).

Generated automatically with:
$ clang-tidy -fix -checks=-*,llvm-qualified-auto $(find
lib/Target/AMDGPU/ -type f)