compiler: do not perform nil checking when indexing slices

The x/tools/go/ssa package splits slice loads/stores into two
operations. So for code like this:

    x = p[3]

It has two instructions:

    x_ptr = &p[3]
    x = *x_ptr

This makes the IR simpler, but also means we're accidentally inserting
more nil checks than necessary: the slice index operation has
effectively already checked for nil by performing a bounds check.
Therefore, omit nil pointer checks for pointers created by
*ssa.IndexAddr.

This change is necessary to make sure a future removal of runtime.isnil
will not cause the escape analysis pass to regress. Apart from that, it
reduces code size slightly in many smoke tests (with no increases in
code size).

compiler/compiler.go

@@ -1154,7 +1154,7 @@ func (b *builder) createInstruction(instr ssa.Instruction) {
 	case *ssa.Store:
 		llvmAddr := b.getValue(instr.Addr)
 		llvmVal := b.getValue(instr.Val)
-		b.createNilCheck(llvmAddr, "store")
+		b.createNilCheck(instr.Addr, llvmAddr, "store")
 		if b.targetData.TypeAllocSize(llvmVal.Type()) == 0 {
 			// nothing to store
 			return
@@ -1402,7 +1402,7 @@ func (b *builder) createFunctionCall(instr *ssa.CallCommon) (llvm.Value, error)
 		// This is a func value, which cannot be called directly. We have to
 		// extract the function pointer and context first from the func value.
 		callee, context = b.decodeFuncValue(value, instr.Value.Type().Underlying().(*types.Signature))
-		b.createNilCheck(callee, "fpcall")
+		b.createNilCheck(instr.Value, callee, "fpcall")
 	}
 
 	var params []llvm.Value
@@ -1548,7 +1548,7 @@ func (b *builder) createExpr(expr ssa.Value) (llvm.Value, error) {
 		// > For an operand x of type T, the address operation &x generates a
 		// > pointer of type *T to x. [...] If the evaluation of x would cause a
 		// > run-time panic, then the evaluation of &x does too.
-		b.createNilCheck(val, "gep")
+		b.createNilCheck(expr.X, val, "gep")
 		// Do a GEP on the pointer to get the field address.
 		indices := []llvm.Value{
 			llvm.ConstInt(b.ctx.Int32Type(), 0, false),
@@ -1596,7 +1596,7 @@ func (b *builder) createExpr(expr ssa.Value) (llvm.Value, error) {
 				// > generates a pointer of type *T to x. [...] If the
 				// > evaluation of x would cause a run-time panic, then the
 				// > evaluation of &x does too.
-				b.createNilCheck(bufptr, "gep")
+				b.createNilCheck(expr.X, bufptr, "gep")
 			default:
 				return llvm.Value{}, b.makeError(expr.Pos(), "todo: indexaddr: "+typ.String())
 			}
@@ -2588,7 +2588,7 @@ func (b *builder) createUnOp(unop *ssa.UnOp) (llvm.Value, error) {
 			}
 			return b.CreateBitCast(fn, b.i8ptrType, ""), nil
 		} else {
-			b.createNilCheck(x, "deref")
+			b.createNilCheck(unop.X, x, "deref")
 			load := b.CreateLoad(x, "")
 			return load, nil
 		}