cmd/compile: allow composite literal structs with _ fields
Given code such as
type T struct {
_ string
}
func f() {
var x = T{"space"}
// ...
}
the compiler rewrote the 'var x' line as
var x T
x._ = "space"
The compiler then rejected the assignment to
a blank field, thus rejecting valid code.
It also failed to catch a number of invalid assignments.
And there were insufficient checks for validity
when emitting static data, leading to ICEs.
To fix, check earlier for explicit blanks field names,
explicitly handle legit blanks in sinit,
and don't try to emit static data for nodes
for which typechecking has failed.