cmd/compile: avoid stack temporary for struct literal assignment to addressable targets

[chabbimilind]

When assigning a struct/array literal to a non-local addressable target
(slice index, global, pointer deref, struct field through pointer), the
compiler previously created a stack temporary, zeroed it, filled fields,
then bulk-copied to the destination. This was because oaslit() required
LHS to be isSimpleName() (stack-local variable).

Extend oaslit with oaslitAddr, which takes the address of the destination
once and decomposes the literal into direct field stores through that
pointer. For partially-initialized literals, the destination is zeroed
in-place before filling fields, eliminating the bulk copy entirely.

Safety is ensured by compLitFieldsSafe/exprSafeForDirectStore, which
conservatively accept only constants, nil, and stack-local non-addrtaken
names as RHS values -- these cannot alias with any addressable destination.
When nested struct literals are partially initialized, compLitAllFieldsSet
detects this and ensures the destination is zeroed before field stores.

Benchmark results (arm64, Apple M1 Max):

                             |  baseline   |           optimized            |
                             |   sec/op    |   sec/op     vs base           |

StructLitAssign/SliceLiteral 8.91n +/- 1% 4.57n +/- 2% -48.75% (p=0.000)
StructLitAssign/Global 8.04n +/- 1% 3.97n +/- 1% -50.55% (p=0.000)
StructLitAssign/PtrDeref 8.22n +/- 0% 3.84n +/- 1% -53.37% (p=0.000)
StructLitAssign/Nested 3.99n +/- 1% 2.93n +/- 2% -26.62% (p=0.000)
StructLitAssign/Embedded 4.01n +/- 1% 2.91n +/- 1% -27.32% (p=0.000)
StructLitAssign/FuncCallRHS 9.17n +/- 1% 5.62n +/- 1% -38.75% (p=0.000)
StructLitAssign/PartialHalf 5.79n +/- 1% 4.12n +/- 1% -28.74% (p=0.000)
StructLitAssign/AliasRHS 7.93n +/- 1% 7.94n +/- 2% ~ (no regression)

Test plan:

• go test cmd/compile/... -count=1 -- all pass
• go test cmd/internal/testdir -run Test -count=1 -- asmcheck tests pass
• ./all.bash -- all pass (only pre-existing TestFortran failure due to local gfortran arch mismatch)
• Benchmarks with benchstat -count=8 -benchtime=5s show 27-53% improvement, no regressions

Fixes #78597

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Patch Set 1:

(1 comment)

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Patch Set 2:

(2 comments)

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