cmd/compile: inline small static initializers

[randall77]

type T struct {
	a, b, c, d int
}
func f(p *T) {
	*p = T{1, 2, 3, 4}
}

The code for f is:

(tmp1.go:8)	MOVQ	"".statictmp_0(SB), AX
(tmp1.go:8)	MOVQ	"".statictmp_0+8(SB), CX
(tmp1.go:8)	MOVQ	"".statictmp_0+16(SB), DX
(tmp1.go:8)	MOVQ	"".statictmp_0+24(SB), BX
(tmp1.go:8)	MOVQ	"".p+8(FP), SI
(tmp1.go:8)	MOVQ	AX, (SI)
(tmp1.go:8)	MOVQ	CX, 8(SI)
(tmp1.go:8)	MOVQ	DX, 16(SI)
(tmp1.go:8)	MOVQ	BX, 24(SI)

That's kind of dumb to copy from a statictmp. Just use MOVQ $1, (SI) and so forth.

[josharian]

This was broken by the fix to #12841. The trick here is to figure out how to selectively disable that optimization so that #12841 doesn't regress.

[josharian]

CL shortly.

[gopherbot]

CL https://golang.org/cl/43154 mentions this issue.