go/types no longer reports a constant value for a slice,
e.g. []byte("foo").
ssa.Const no longer support slice constants, only
bool/string/numeric and nil.
emitConv emits a Convert instruction for string->[]byte
conversions.
Added regression test for bug 6949.
R=gri
CC=golang-codereviews
https://golang.org/cl/50590043