Restore AllocOptPass in the -O1 scalar optimizer pipeline

[rbvermaa]

What

Backports the else if (O.getSpeedupLevel() >= 1) branch of buildScalarOptimizerPipeline that upstream added in JuliaLang/julia#52850 — "Add passes to -O1 pipeline to reduce allocations in reinterpret".

Why

That branch was backported to release-1.10 (#57731) and release-1.11 (#57732) on 2025-03-11, but never landed on the 1.12 line:

Ref	else if (>= 1) block
v1.11.9	present
master	present
release-1.13	present
v1.12.6	absent
release-1.12	absent
backports-release-1.12	absent

We found no revert on master and no stated rationale for the omission. #52850 merged 2025-02-21 and v1.12.0-rc1 was tagged 2025-07-12, so a branch cut after the merge should have inherited it.

Effect

Without AllocOptPass at -O1, a Ref holding an immutable value which never escapes and is used only via pointer_from_objref is heap-allocated:

struct TwoWords; a::UInt64; b::UInt64; end

@noinline function load_first_word(x)
    r = Ref(x); v = UInt64(0)
    GC.@preserve r begin
        p = reinterpret(Ptr{UInt64}, pointer_from_objref(r))
        v = unsafe_load(p)
    end
    return v
end

Build	-O1	-O2
1.10.11	0	0
1.11.9	0	0
1.12.6	32	0
1.12.0-rc2+RAI	32	0
1.12.6+RAI	32	0

Reproduces on stock and +RAI builds, on aarch64-darwin and x86_64-linux. In the IR the box is a real @ijl_gc_small_alloc at -O1 and absent at -O2.

raicode runs its test suite at -O1 (JULIA_COMMON_ARGS) and asserts several hot paths allocate nothing — TypeNormalization's byte-string hash and fast_reinterpret use exactly this idiom deliberately to avoid allocating. So -O1 currently gives no usable "this code does not allocate" signal. Production builds run at -O2/-O3 and are unaffected.

Risk

The added pass list is byte-identical to v1.11.9's. All six passes are already used elsewhere in src/pipeline.cpp and their headers are already included, so this adds no new dependencies. The >= 2 branch is untouched, so -O2/-O3 codegen is unchanged.

Not compiled locally (no build tree) — validation is via raicode's deps.julia.x86_64-linux build plus the -O1 allocation assertions that currently fail.