tests: rustc_public: Check const allocation for all variables (1 of 11 was missing)

[Enselic]

In the test tests/ui-fulldeps/rustc_public/check_allocation.rs there is a check for constant allocations of local variables of this function:

fn other_consts() {{
    let _max_u128 = u128::MAX;
    let _min_i128 = i128::MIN;
    let _max_i8 = i8::MAX;
    let _char = 'x';
    let _false = false;
    let _true = true;
    let _ptr = &BAR;
    let _null_ptr: *const u8 = NULL;
    let _tuple = TUPLE;
    let _char_id = const {{ type_id::<char>() }};
    let _bool_id = const {{ type_id::<bool>() }};
}}

The current test only finds 10 out of 11 allocations. The constant allocation for

let _ptr = &BAR;

is not checked, because the SingleUseConsts MIR pass does not optimize away that assignment. Add code to also collect constant allocation from assignment rvalues to find the constant allocation for that last variable.

Not only does this change make sense on its own, it also makes the test pass both with and without the SingleUseConsts pass.

Discovered while investigating ways to avoid this tests/ui-fulldeps/rustc_public/check_allocation.rs hack from #151426 which wants to stop running SingleUseConsts for non-optimized builds.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

r? compiler

[chenyukang]

@bors reroll

[chenyukang]

@rustbot reroll

[chenyukang]

oops, assign the same reviwer

@rustbot reroll

[jieyouxu]

r? me