[AMDGPU] Add IsSingle to a few Interp instructions (llvm#95984)

A _e64 suffix should not be printed since these instructions only have
one legal encoding length. The absence of the IsSingle flag is hidden by
how the string is printed. We could fix it for GFX10 as well, but we
shouldn't change the asm output to omit _e64 at this point.
NFC.

Author: Sisyph

llvm/lib/Target/AMDGPU/VOP3Instructions.td

@@ -107,7 +107,7 @@ class getInterp16Ins <bit HasSrc2, bit HasOMod,
 }
 
 class VOP3_INTERP16 <list<ValueType> ArgVT> : VOPProfile<ArgVT> {
-
+  let IsSingle = 1;
   let HasOMod = !ne(DstVT.Value, f16.Value);
   let HasHigh = 1;