cmd/compile: optimize struct reset logic

[dsnet]

Consider the following:

type S struct {
	Large [8]int
	Small int
}

var s = new(S)

func BenchmarkResetA(b *testing.B) {
	for i := 0; i < b.N; i++ {
		*s = S{Large: s.Large}
	}
}

func BenchmarkResetB(b *testing.B) {
	for i := 0; i < b.N; i++ {
		s.Small = 0
	}
}

Performance:

BenchmarkResetA
BenchmarkResetA-24    	131323767	         9.117 ns/op
BenchmarkResetB
BenchmarkResetB-24    	1000000000	         0.2167 ns/op

Both snippets are semantically equivalent, but ResetB is much faster.
However, ResetA is common in reset logic where the intention is to reset all fields except a few select fields. This pattern is more maintainable since it properly resets the struct even if new fields are added to it later on.

The compiler should recognize the pattern exhibited in ResetA and avoid the unnecessary copy of s.Large = s.Large.

[mvdan]

dup of #24386?

[thanm]

Also possible related, #24416