cmd/compile: add generic rules to eliminate some unnecessary stores

Eliminates stores of values that have just been loaded from the same
location. Handles the common case where there are up to 3 intermediate
stores to non-overlapping struct fields.

For example the loads and stores of x.a, x.b and x.d in the following
function are now removed:

type T struct {
	a, b, c, d int
}

func f(x *T) {
	y := *x
	y.c += 8
	*x = y
}

Before this CL (s390x):

TEXT    "".f(SB)
	MOVD    "".x(R15), R5
	MOVD    (R5), R1
	MOVD    8(R5), R2
	MOVD    16(R5), R0
	MOVD    24(R5), R4
	ADD     $8, R0, R3
	STMG    R1, R4, (R5)
	RET

After this CL (s390x):

TEXT	"".f(SB)
	MOVD	"".x(R15), R1
	MOVD	16(R1), R0
	ADD	$8, R0, R0
	MOVD	R0, 16(R1)
	RET

In total these rules are triggered ~5091 times during all.bash,
which is broken down as:

Intermediate stores | Triggered
--------------------+----------
0                   | 1434
1                   | 2508
2                   | 888
3                   | 261
--------------------+----------

Change-Id: Ia4721ae40146aceec1fdd3e65b0e9283770bfba5
Reviewed-on: https://go-review.googlesource.com/38793


Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>