[IntrinsicEmitter] Extend argument overloading with forward references.

Extend the mechanism to overload intrinsic arguments by using either
backward or forward references to the overloadable arguments.

In for example:

  def int_something : Intrinsic<[LLVMPointerToElt<0>],
                                [llvm_anyvector_ty], []>;

LLVMPointerToElt<0> is a forward reference to the overloadable operand
of type 'llvm_anyvector_ty' and would allow intrinsics such as:

  declare i32* @llvm.something.v4i32(<4 x i32>);
  declare i64* @llvm.something.v2i64(<2 x i64>);

where the result pointer type is deduced from the element type of the
first argument.

If the returned pointer is not a pointer to the element type, LLVM will
give an error:

  Intrinsic has incorrect return type!
  i64* (<4 x i32>)* @llvm.something.v4i32

Reviewers: RKSimon, arsenm, rnk, greened

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D62995

llvm-svn: 363233