cmd/compile: optimize math/bits.Div32 for arm64

cmd/compile: optimize math/bits.Div32 for arm64

Benchmark:
name     old time/op  new time/op  delta
Div-8    22.0ns ± 0%  22.0ns ± 0%     ~     (all equal)
Div32-8  6.51ns ± 0%  3.00ns ± 0%  -53.90%  (p=0.000 n=10+8)
Div64-8  22.5ns ± 0%  22.5ns ± 0%     ~     (all equal)

Code:
func div32(hi, lo, y uint32) (q, r uint32) {return bits.Div32(hi, lo, y)}

Before:
        0x0020 00032 (test.go:24)       MOVWU   "".y+8(FP), R0
        0x0024 00036 ($GOROOT/src/math/bits/bits.go:472)        CBZW    R0, 132
        0x0028 00040 ($GOROOT/src/math/bits/bits.go:472)        MOVWU   "".hi(FP), R1
        0x002c 00044 ($GOROOT/src/math/bits/bits.go:472)        CMPW    R1, R0
        0x0030 00048 ($GOROOT/src/math/bits/bits.go:472)        BLS     96
        0x0034 00052 ($GOROOT/src/math/bits/bits.go:475)        MOVWU   "".lo+4(FP), R2
        0x0038 00056 ($GOROOT/src/math/bits/bits.go:475)        ORR     R1<<32, R2, R1
        0x003c 00060 ($GOROOT/src/math/bits/bits.go:476)        CBZ     R0, 140
        0x0040 00064 ($GOROOT/src/math/bits/bits.go:476)        UDIV    R0, R1, R2
        0x0044 00068 (test.go:24)       MOVW    R2, "".q+16(FP)
        0x0048 00072 ($GOROOT/src/math/bits/bits.go:476)        UREM    R0, R1, R0
        0x0050 00080 (test.go:24)       MOVW    R0, "".r+20(FP)
        0x0054 00084 (test.go:24)       MOVD    -8(RSP), R29
        0x0058 00088 (test.go:24)       MOVD.P  32(RSP), R30
        0x005c 00092 (test.go:24)       RET     (R30)

After:
        0x001c 00028 (test.go:24)       MOVWU   "".y+8(FP), R0
        0x0020 00032 (test.go:24)       CBZW    R0, 92
        0x0024 00036 (test.go:24)       MOVWU   "".hi(FP), R1
        0x0028 00040 (test.go:24)       CMPW    R0, R1
        0x002c 00044 (test.go:24)       BHS     84
        0x0030 00048 (test.go:24)       MOVWU   "".lo+4(FP), R2
        0x0034 00052 (test.go:24)       ORR     R1<<32, R2, R4
        0x0038 00056 (test.go:24)       UDIV    R0, R4, R3
        0x003c 00060 (test.go:24)       MSUB    R3, R4, R0, R4
        0x0040 00064 (test.go:24)       MOVW    R3, "".q+16(FP)
        0x0044 00068 (test.go:24)       MOVW    R4, "".r+20(FP)
        0x0048 00072 (test.go:24)       MOVD    -8(RSP), R29
        0x004c 00076 (test.go:24)       MOVD.P  16(RSP), R30
        0x0050 00080 (test.go:24)       RET     (R30)

UREM instruction in the previous assembly code will be converted to UDIV and MSUB instructions
on arm64. However the UDIV instruction in UREM is unnecessary, because it's a duplicate of the
previous UDIV. This CL adds a rule to have this extra UDIV instruction removed by CSE.

Change-Id: Ie2508784320020b2de022806d09f75a7871bb3d7
Reviewed-on: https://go-review.googlesource.com/c/159577
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>