librustc: Expand constant expressions before -premote-returned-structures.

In the presence of cycles, removeDeadConstantUsers won't work. Since we can't
use mutateType+RAUW on constants, we must expand constant expressions beforehand.
librustc: Remove unused file.
Mk/crates.mk: Remove libsimd && libsimd_syntax. As of this commit, the code is
still present, it's just not built.
LLVM: Bump commit.