cmd/compile/internal/ssa: elide redundant 32-bit zero extensions on arm64

[andig]

ZeroUpper32Bits knows about arm64's W-form ALU results, but not about its
zero-extending loads and register moves. The late lower rule
(MOVWUreg x) && ZeroUpper32Bits(x, 3) => x therefore never fires for a
value that came from MOVWUload/MOVHUload/MOVBUload or from a narrower
MOV*Ureg.

With a single use the extension still folds away, via
(MOVWUreg x:(MOVWUload _ _)) => (MOVDreg x) and
(MOVDreg x) && x.Uses == 1 => (MOVDnop x). With a second consumer -- a
loaded uint32 field used both as a slice index and in a bounds compare,
the regexp onepass shape -- the MOVDreg survives as a real MOVD Rx,Ry
register copy.

Add the arm64 zero-extending loads and register moves to
ZeroUpper32Bits, mirroring the byte and halfword op lists that CL 804064
adds to ZeroUpper56Bits and ZeroUpper48Bits. The two changes are
independent; this one applies on its own.

Measured at 7b4aab8 with go build -a -gcflags=-S std at
GOARCH=arm64: 117 fewer register copies (runtime -36, regexp -26,
internal/zstd -11, internal/coverage/cformat -8) for -592 bytes of
function text across 26 packages, with a single-copy regression in
errors.

Updates #43357

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This CL has failed the run. Reason:

Tryjob golang/try/gotip-windows-386 has failed with summary (view all results):

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• go on master (change 805860)

To reproduce, try gomote repro 8675248901453639969.

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