Because PPC lacks store-immediate, remove the instruction
that implies that it exists.  Replace it with storezero for
the special case of storing zero, because R0 is reserved zero
for Go (though the assembler knows this, do it in SSA).

Also added address folding for storezero.
(Now corrected to use right-sized stores in bulk-zero code.)

Hello.go now compiles to
genssa main
    00000 (...hello.go:7) TEXT "".main(SB), $0
    00001 (...hello.go:7) FUNCDATA $0, "".gcargs·0(SB)
    00002 (...hello.go:7) FUNCDATA $1, "".gclocals·1(SB)
v23 00003 (...hello.go:8) MOVD $go.string."Hello, World!\n"(SB), R3
v11 00004 (...hello.go:8) MOVD R3, 32(R1)
v22 00005 (...hello.go:8) MOVD $14, R3
v6  00006 (...hello.go:8) MOVD R3, 40(R1)
v20 00007 (...hello.go:8) MOVD R0, 48(R1)
v18 00008 (...hello.go:8) MOVD R0, 56(R1)
v9  00009 (...hello.go:8) MOVD R0, 64(R1)
v10 00010 (...hello.go:8) CALL fmt.Printf(SB)
b2  00011 (...hello.go:9) RET
    00012 (<unknown line number>) END

Updates #16010

Change-Id: I33cfd98c21a1617502260ac753fa8cad68c8d85a
Reviewed-on: https://go-review.googlesource.com/25151Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>