cmd/compile: set correct type for OpIData

Since CL 270057, there're many attempts to fix the expand_calls pass
with interface{}-typed. But all of them did not fix the root cause. The
main issue is during SSA conversion in gc/ssa.go, for empty interface
case, we make its type as n.Type, instead of BytePtr.

To fix these, we can just use BytePtr for now, since when itab fields
are treated as scalar.

No significal changes on compiler speed, size.

cmd/compile/internal/ssa
expandCalls.func6 9488 -> 9232  (-2.70%)

file                       before   after    Δ       %
cmd/compile/internal/ssa.s 3992893  3992637  -256    -0.006%
total                      20500447 20500191 -256    -0.001%

Fixes #43112
Updates #42784
Updates #42727
Updates #42568

Change-Id: I0b15d9434e0be5448453e61f98ef9c2d6cd93792
Reviewed-on: https://go-review.googlesource.com/c/go/+/276952


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>