[Mem2Reg] Always allow single-store optimization for dominating stores

In llvm#97711 the single-store optimization was disabled for the case
where the value is potentially poison, as this may produce incorrect
results for loads of uninitialized memory.

However, this resulted in compile-time regressions. Address these
by still allowing the single-store optimization to occur in cases
where the store dominates the load, as we know that such a load
will always read initialized memory.

(cherry picked from commit daaea12)

[Mem2Reg] Always allow single-store optimization for dominating stores

In llvm#97711 the single-store optimization was disabled for the case
where the value is potentially poison, as this may produce incorrect
results for loads of uninitialized memory.

However, this resulted in compile-time regressions. Address these
by still allowing the single-store optimization to occur in cases
where the store dominates the load, as we know that such a load
will always read initialized memory.

(cherry picked from commit daaea12)

[Mem2Reg] Always allow single-store optimization for dominating stores

In llvm#97711 the single-store optimization was disabled for the case
where the value is potentially poison, as this may produce incorrect
results for loads of uninitialized memory.

However, this resulted in compile-time regressions. Address these
by still allowing the single-store optimization to occur in cases
where the store dominates the load, as we know that such a load
will always read initialized memory.

(cherry picked from commit daaea12)

Merge remote-tracking branch 'upstream/release/18.x' into rustc/18.1-…

…2024-05-19

LLVM 18.1.3 Release

Merge pull request llvm#172 from nikic/rustc/18.0-2024-02-13

Update to LLVM 18.1.2

LLVM 18.1.2 Release

LLVM 18.1.1 Release

LLVM 18.1.0 Release

LLVM 18.1.0-rc4 Release