cmd/compile: change the way SSA does slice zero-cap detection

cmd/compile: change the way SSA does slice zero-cap detection

There is a special case for slicing s[i:j] when the resulting
slice has zero capacity, to prevent pointing to the next object
in memory.

Change this special case code from:
  rptr := rcap == 0 ? ptr : ptr+i*elemsize
to
  rptr := ptr + (rcap == 0 ? 0 : i) * elemsize

This change leads to slightly smaller generated code, replacing
a load with a register zero.

old:
	0x002e 00046 (slice.go:8)	CMPQ	BX, $0
	0x0032 00050 (slice.go:8)	JEQ	$0, 78
	0x0034 00052 (slice.go:8)	MOVQ	"".a+8(FP), BP
	0x0039 00057 (slice.go:8)	LEAQ	(BP)(CX*8), AX
	0x003e 00062 ... rest of function ...

	0x004e 00078 (slice.go:7)	MOVQ	"".a+8(FP), AX
	0x0053 00083 (slice.go:8)	JMP	62

new:
	0x002e 00046 (slice.go:8)	CMPQ	BX, $0
	0x0032 00050 (slice.go:8)	JEQ	$0, 78
	0x0034 00052 (slice.go:8)	MOVQ	"".a+8(FP), BP
	0x0039 00057 (slice.go:8)	LEAQ	(BP)(CX*8), AX
	0x003e 00062 ... rest of function...

	0x004e 00078 (slice.go:8)	MOVQ	$0, CX
	0x0050 00080 (slice.go:8)	JMP	52

Change-Id: I2a396616b0d7b090c226a47c92a7ba14b128401f
Reviewed-on: https://go-review.googlesource.com/20994
Reviewed-by: David Chase <drchase@google.com>